Genomic imprinting is an epigenetic phenomenon by which certain genes can be expressed in a parent-of-origin-specific manner. It may also ensure transposable elements remain epigenetically silenced throughout gametogenic reprogramming to maintain genome integrity. It is an inheritance process independent of the classical Mendelian inheritance. In Homo sapiens, imprinted alleles are silenced such that the genes are either expressed only from the non-imprinted allele inherited from the mother (e.g. H19 or CDKN1C), or in other instances from the non-imprinted allele inherited from the father (e.g. IGF-2). However, in plants parental genomic imprinting can refer to gene expression both solely or primarily from either parent's allele. Forms of genomic imprinting have been demonstrated in fungi, plants and animals.
Genomic imprinting is an epigenetic process that can involve DNA methylation and histone modulation in order to achieve monoallelic gene expression without altering the genetic sequence. These epigenetic marks are established in the germline and can be maintained through mitotic divisions.
Appropriate expression of imprinted genes is important for normal development, with numerous genetic diseases associated with imprinting defects including Beckwith–Wiedemann syndrome, Silver–Russell syndrome, Angelman syndrome and Prader–Willi syndrome.
- 1 Overview
- 2 Imprinted genes in mammals
- 3 Examples
- 4 Imprinted genes in plants
- 5 See also
- 6 References
- 7 External links
In diploid organisms, somatic cells possess two copies of the genome. Each autosomal gene is therefore represented by two copies, or alleles, with one copy inherited from each parent at fertilisation. For the vast majority of autosomal genes, expression occurs from both alleles simultaneously. In mammals, however, a small proportion (<1%) of genes are imprinted, meaning that gene expression occurs from only one allele. The expressed allele is dependent upon its parental origin. For example, the gene encoding Insulin-like growth factor 2 (IGF2/Igf2) is only expressed from the allele inherited from the father.
The term "imprinting" was first used to describe events in the insect Pseudococcus nipae. In Pseudococcids (mealybugs) (Homoptera, Coccoidea) both the male and female develop from a fertilised egg. In females, all chromosomes remain euchromatic and functional. In embryos destined to become males, one haploid set of chromosomes becomes heterochromatinised after the sixth cleavage division and remains so in most tissues; males are thus functionally haploid. In insects, imprinting describes the silencing of the paternal genome in males, and thus is involved in sex determination. In mammals, genomic imprinting describes the processes involved in introducing functional inequality between two parental alleles of a gene.
Imprinted genes in mammals
That imprinting might be a feature of mammalian development was suggested in breeding experiments in mice carrying reciprocal translocations. Nucleus transplantation experiments in mouse zygotes in the early 1980s confirmed that normal development requires the contribution of both the maternal and paternal genomes. The vast majority of mouse parthenogenones/gynogenones (with two maternal or egg genomes) and androgenones (with two paternal or sperm genomes) die at, or before, the blastocyst/implantation stage. In the rare instances that they develop to postimplantation stages, gynogenetic embryos show better embryonic development relative to placental development, while for androgenones, the reverse is true. Nevertheless, for the latter, only a few have been described.
Parthenogenetic/gynogenetic embryos have twice the normal expression level of maternally derived genes, and lack expression of paternally expressed genes, while the reverse is true for androgenetic embryos. It is now known that there are at least 80 imprinted genes in humans and mice, many of which are involved in embryonic and placental growth and development. Various methods have been used to identify imprinted genes. In swine, Bischoff et al. 2009 compared transcriptional profiles using short-oligonucleotide microarrays (Affymetrix Porcine GeneChip) to survey differentially expressed genes between parthenotes (2 maternal genomes) and control fetuses (1 maternal, 1 paternal genome) An intriguing study surveying the transcriptome of murine brain tissues revealed over 1300 imprinted gene loci (approximately 10-fold more than previously reported) by Illumina RNA-sequencing (RNA-Seq) technology from F1 hybrids resulting from reciprocal crosses. The result however has been challenged by others who claimed that this is an overestimation by an order of magnitude due to flawed statistical analysis.
No naturally occurring cases of parthenogenesis exist in mammals because of imprinted genes. Experimental manipulation of a paternal methylation imprint controlling the Igf2 gene has, however, recently allowed the creation of rare individual mice with two maternal sets of chromosomes - but this is not a true parthenogenone. Hybrid offspring of two species may exhibit unusual growth due to the novel combination of imprinted genes.
Genetic mapping of imprinted genes
At the same time as the generation of the gynogenetic and androgenetic embryos discussed above, mouse embryos were also being generated that contained only small regions that were derived from either a paternal or maternal source. The generation of a series of such uniparental disomies, which together span the entire genome, allowed the creation of an imprinting map. Those regions which when inherited from a single parent result in a discernible phenotype contain imprinted gene(s). Further research showed that within these regions there were often numerous imprinted genes. Around 80% of imprinted genes are found in clusters such as these, called imprinted domains, suggesting a level of co-ordinated control. More recently, genome-wide screens to identify imprinted genes have used differential expression of mRNAs from control fetuses and parthenogenetic or androgenetic fetuses hybridized to expression arrays, allele-specific gene expression using SNP genotyping arrays, transcriptome sequencing, and in silico prediction pipelines.
Imprinting is a dynamic process. It must be possible to erase and re-establish imprints through each generation so that genes that are imprinted in an adult may still be expressed in that adult's offspring. (ex. The maternal genes that control insulin production will be imprinted in a male but will be expressed in any of the male's offspring that inherit these genes). The nature of imprinting must therefore be epigenetic rather than DNA sequence dependent. In germline cells the imprint is erased and then re-established according to the sex of the individual, i.e. in the developing sperm (during spermatogenesis), a paternal imprint is established, whereas in developing oocytes (oogenesis), a maternal imprint is established. This process of erasure and reprogramming is necessary such that the germ cell imprinting status is relevant to the sex of the individual. In both plants and mammals there are two major mechanisms that are involved in establishing the imprint; these are DNA methylation and histone modifications.
The grouping of imprinted genes within clusters allows them to share common regulatory elements, such as non-coding RNAs and differentially methylated regions (DMRs). When these regulatory elements control the imprinting of one or more genes, they are known as imprinting control regions (ICR). The expression of non-coding RNAs, such as Air on mouse chromosome 17 and KCNQ1OT1 on human chromosome 11p15.5, have been shown to be essential for the imprinting of genes in their corresponding regions.
Differentially methylated regions are generally segments of DNA rich in cytosine and guanine nucleotides, with the cytosine nucleotides methylated on one copy but not on the other. Contrary to expectation, methylation does not necessarily mean silencing; instead, the effect of methylation depends upon the default state of the region.
Functions of imprinted genes
The control of expression of specific genes by genomic imprinting is unique to therian mammals (placental mammals and marsupials) and flowering plants. Imprinting of whole chromosomes has been reported in mealybugs (Genus: Pseudococcus). and a fungus gnat (Sciara). It has also been established that X-chromosome inactivation occurs in an imprinted manner in the extra-embryonic tissues of mice and all tissues in marsupials, where it is always the paternal X-chromosome which is silenced.
The majority of imprinted genes in mammals have been found to have roles in the control of embryonic growth and development, including development of the placenta. Other imprinted genes are involved in post-natal development, with roles affecting suckling and metabolism.
Theories on the origins of imprinting
A widely accepted hypothesis for the evolution of genomic imprinting is the "parental conflict hypothesis." Also known as the kinship theory of genomic imprinting, this hypothesis states that the inequality between parental genomes due to imprinting is a result of the differing interests of each parent in terms of the evolutionary fitness of their genes. The father's genes that encode for imprinting gain greater fitness through the success of the offspring, at the expense of the mother. The mother's evolutionary imperative is often to conserve resources for her own survival while providing sufficient nourishment to current and subsequent litters. Accordingly, paternally expressed genes tend to be growth promoting whereas maternally expressed genes tend to be growth limiting. In support of this hypothesis, genomic imprinting has been found in all placental mammals, where post-fertilisation offspring resource consumption at the expense of the mother is high; although it has also been found in oviparous birds  where there is relatively little post-fertilisation resource transfer and therefore less parental conflict.
However, our understanding of the molecular mechanisms behind genomic imprinting show that it is the maternal genome that controls much of the imprinting of both its own and the paternally-derived genes in the zygote, making it difficult to explain why the maternal genes would willingly relinquish their dominance to that of the paternally-derived genes in light of the conflict hypothesis. Several other hypotheses that propose a coadaptive reason for the evolution of genomic imprinting have been proposed.
Others have approached their study of the origins of genomic imprinting from a different side, arguing that natural selection is operating on the role of epigenetic marks as machinery for homologous chromosome recognition during meiosis, rather than on their role in differential expression. This argument centers on the existence of epigenetic effects on chromosomes that do not directly affect gene expression, but do depend on which parent the chromosome originated from. This group of epigenetic changes that depend on the chromosome's parent of origin (including both those that affect gene expression and those that do not) are called parental origin effects, and include phenomena such as paternal X inactivation in the marsupials, nonrandom parental chromatid distribution in the ferns, and even mating type switching in yeast. This diversity in organisms that show parental origin effects has prompted theorists to place the evolutionary origin of genomic imprinting before the last common ancestor of plants and animals, over a billion years ago.
Natural selection for genomic imprinting requires genetic variation in a population. A hypothesis for the origin of this genetic variation states that the host-defense system responsible for silencing foreign DNA elements, such as genes of viral origin, mistakenly silenced genes whose silencing turned out to be beneficial for the organism. There appears to be an over-representation of retrotransposed genes, that is to say genes that are inserted into the genome by viruses, among imprinted genes. It has also been postulated that if the retrotransposed gene is inserted close to another imprinted gene, it may just acquire this imprint.
Problems associated with imprinting
Imprinting may cause problems in cloning, with clones having DNA that is not methylated in the correct position. It is possible that this is due to a lack of time for reprogramming to be completely achieved. When a nucleus is added to an egg during somatic cell nuclear transfer, the egg starts dividing in minutes, as compared to the days or months it takes for reprogramming during embryonic development. If time is the responsible factor, it may be possible to delay cell division in clones, giving time for proper reprogramming to occur.
An allele of the "callipyge" (from the Greek for "beautiful buttocks"), or CLPG, gene in sheep produces large buttocks consisting of muscle with very little fat. The large-buttocked phenotype only occurs when the allele is present on the copy of chromosome 18 inherited from a sheep's father and is not on the copy of chromosome 18 inherited from that sheep's mother.
The first imprinted genetic disorders to be described in humans were the reciprocally inherited Prader-Willi syndrome and Angelman syndrome. Both syndromes are associated with loss of the chromosomal region 15q11-13 (band 11 of the long arm of chromosome 15). This region contains the paternally expressed genes (SNRPN and NDN) and the maternally expressed gene (UBE3A).
- Paternal inheritance of a deletion of this region is associated with Prader-Willi syndrome (characterised by hypotonia, obesity, and hypogonadism).
- Maternal inheritance of the same deletion is associated with Angelman syndrome (characterised by epilepsy, tremors, and a perpetually smiling facial expression).
NOEY2 is a paternally expressed imprinted gene located on chromosome 1 in humans. Loss of NOEY2 expression is linked to an increased risk of ovarian and breast cancers; in 41% of breast and ovarian cancers the protein encoded by NOEY2 is not expressed, suggesting that it functions as a tumor suppressor gene Therefore, if a person inherits both chromosomes from the mother, the gene will not be expressed and the individual is put at a greater risk for breast and ovarian cancer.
Imprinted genes in plants
A similar imprinting phenomenon has also been described in flowering plants (angiosperms). During fertilisation of the egg cell, a second, separate fertilization event gives rise to the endosperm, an extraembryonic structure that nourishes the embryo in a manner analogous to the mammalian placenta. Unlike the embryo, the endosperm is often formed from the fusion of two maternal cells with a male gamete. This results in a triploid genome. The uneven ratio of maternal to paternal genomes appears to be critical for seed development. Some genes are found to be expressed from both maternal genomes while others are expressed exclusively from the lone paternal copy.
- Wollmann,H. and F. Berger (2012). "Epigenetic reprogramming during plantreproduction and seed development." Current Opinion in Plant Biology15(1): 63-69
- Mosher, R. A. and C. W. Melnyk (2010). "siRNAs and DNA methylation: seedy epigenetics." Trends in Plant Science 15(4): 204-210
- Martienssen, R. A. and V. Colot (2001). "DNA Methylation and Epigenetic Inheritance in Plants and Filamentous Fungi." Science 293(5532): 1070-1074.
- Wilkinson, Lawrence S.; William Davies and Anthony R. Isles (November 2007). "Genomic imprinting effects on brain development and function". Nature Reviews Neuroscience 8 (11): 832–843. doi:10.1038/nrn2235. PMID 17925812. Retrieved 2008-07-01.
- DeChiara, Thomas M.; Elizabeth J. Robertson and Argiris Efstratiadis (February 1991). "Parental imprinting of the mouse insulin-like growth factor II gene". Cell 64 (4): 849–59. doi:10.1016/0092-8674(91)90513-X. PMID 1997210. Retrieved 2008-07-01.
- Schrader, Franz (May 1921). "The chromosomes in Pseudococcus nipæ". Biological Bulletin 40 (5): 259–270. doi:10.2307/1536736. JSTOR 1536736. Retrieved 2008-07-01.
- Brown, S. W.; U. Nur (1964). "Heterochromatic chromosomes in the coccids". Science 145 (3628): 130–136. doi:10.1126/science.145.3628.130. PMID 14171547.
- Hughes-Schrader, S. (1948). "Cytology of coccids (Coccoïdea-Homoptera)". Advances in Genetics. Advances in Genetics 35 (2): 127–203. doi:10.1016/S0065-2660(08)60468-X. ISBN 9780120176021. PMID 18103373.
- Nur, U. (1990). "Heterochromatization and euchromatization of whole genomes in scale insects (Coccoidea: Homoptera)". Dev. Suppl.: 29–34. PMID 2090427.
- Feil, Robert Feil; Frédéric Berger (April 2007). "Convergent evolution of genomic imprinting in plants and mammals". Trends in Genetics 23 (4): 192–9. doi:10.1016/j.tig.2007.02.004. PMID 17316885. Retrieved 2008-07-01.
- Lyon, M. F.; P.H. Glenister (February 1977). "Factors affecting the observed number of young resulting from adjacent-2 disjunction in mice carrying a translocation". Genetics Research 29 (1): 83–92. doi:10.1017/S0016672300017134. PMID 559611.
- Barton, S. C.; Surani, M. A. H. et al. (1984). "Role of paternal and maternal genomes in mouse development". Nature 311 (5984): 374–376. doi:10.1038/311374a0. PMID 6482961.
- Mann, J. R.; R.H. Lovell-Badge (1984). "Inviability of parthenogenones is determined by pronuclei, not egg cytoplasm". Nature 310 (5972): 66–7. doi:10.1038/310066a0. PMID 6738704.
- McGrath, J.; D. Solter (May 1984). "Completion of mouse embryogenesis requires both the maternal and paternal genomes". Cell 37 (1): 179–83. doi:10.1016/0092-8674(84)90313-1. PMID 6722870. Retrieved 2008-07-01.
- Isles, A. R.; A. J. Holland (January 2005). "Imprinted genes and mother-offspring interactions". Early Human Development 81 (1): 73–7. doi:10.1016/j.earlhumdev.2004.10.006. PMID 15707717. Retrieved 2008-07-01.
- Morison, I.M.; J. P. Ramsay and H. G. Spencer (August 2005). "A census of mammalian imprinting". Trends in Genetics 21 (8): 457–65. doi:10.1016/j.tig.2005.06.008. PMID 15990197. Retrieved 2008-07-01.
- Reik, W.; A. Lewis (May 2005). "Co-evolution of X-chromosome inactivation and imprinting in mammals". Nature Reviews Genetics 6 (5): 403–10. doi:10.1038/nrg1602. PMID 15818385.
- Wood, A. J.; R. J. Oakey (November 2006). "Genomic imprinting in mammals: emerging themes and established theories". PLoS Genetics 2 (11): e147. doi:10.1371/journal.pgen.0020147. PMC 1657038. PMID 17121465.
- Bischoff, SR, Tsai, S, Hardison, N, Motsinger-Reif, AA, Freking, BA, Nonneman, D, Rohrer, G, Piedrahita, JA (November 2009). "Characterization of conserved and nonconserved imprinted genes in swine". Biology of Reproduction 81 (5): 906–20. doi:10.1095/biolreprod.109.078139. PMC 2770020. PMID 19571260.
- Gregg, C, Zhang, J, Weissbourd, B, Luo, S, Schroth, GP, Haig, D, Dulac, C (2010). "High-resolution analysis of parent-of-origin allelic expression in the mouse brain". Science 329 (5992): 643–648. doi:10.1126/science.1190830. PMC 3005244. PMID 20616232.
- Erika Check Hayden (2012). "RNA studies under fire". Nature 484 (7395): 428. doi:10.1038/484428a. PMID 22538578.
- Deveale B, van der Kooy D, Babak T (2012). "Critical Evaluation of Imprinted Gene Expression by RNA-Seq: A New Perspective". PLoS Genet 8 (3): e1002600. doi:10.1371/journal.pgen.1002600. PMC 3315459. PMID 22479196.
- "Gene Tug-of-War Leads to Distinct Species". Howard Hughes Medical Institute. 2000-04-30. Retrieved 2008-07-02.
- Cattanach, B. M.; M. Kirk (June 1985). "Differential activity of maternally and paternally derived chromosome regions in mice". Nature 315 (6019): 496–498. doi:10.1038/315496a0. PMID 4000278. Retrieved 2008-07-01.
- McLaughlin, K. J.; P. Szabó, H. Haegel and J. R. Mann (January 1996). "Mouse embryos with paternal duplication of an imprinted chromosome 7 region die at midgestation and lack placental spongiotrophoblast". Development 122 (1): 265–70. PMID 8565838.
- Beechey, Colin; B. M. Cattanach, Andrew Blake and Jo Peters (2008). "Mouse Imprinting Data and References". MRC Harwell. Retrieved 2008-07-02.
- Bartolomei, M. S.; S. M. Tilghman (1997). "Genomic imprinting in mammals". Annual Review of Genetics (subscription required ) 31: 493–525. doi:10.1146/annurev.genet.31.1.493. PMID 9442905.
- Reik, W.; J. Walter (January 2001). "Genomic imprinting: parental influence on the genome". Nature Reviews Genetics 2 (1): 21–32. doi:10.1038/35047554. PMID 11253064.
- Kobayashi, H, Yamada, K, Morita, S, Hiura, H, Fukuda, A, Kagami, M, Ogata, T, Hata, K et al. (May 2009). "Identification of the mouse paternally expressed imprinted gene Zdbf2 on chromosome 1 and its imprinted human homolog ZDBF2 on chromosome 2". Genomics 93 (5): 461–72. doi:10.1016/j.ygeno.2008.12.012. PMID 19200453.
- Bjornsson, HT, Albert, TJ, Ladd-Acosta, CM, Green, RD, Rongione, MA, Middle, CM, Irizarry, RA, Broman, KW, Feinberg, AP (May 2008). "SNP-specific array-based allele-specific expression analysis". Genome Research 18 (5): 771–9. doi:10.1101/gr.073254.107. PMC 2336807. PMID 18369178.
- Babak, T, Deveale, B, Armour, C, Raymond, C, Cleary, MA, Van Der Kooy, D, Johnson, JM, Lim, LP (November 2008). "Global survey of genomic imprinting by transcriptome sequencing". Current biology : CB 18 (22): 1735–41. doi:10.1016/j.cub.2008.09.044. PMID 19026546.
- Luedi, PP, Dietrich, FS, Weidman, JR, Bosko, JM, Jirtle, RL, Hartemink, AJ (December 2007). "Computational and experimental identification of novel human imprinted genes". Genome Research 17 (12): 1723–30. doi:10.1101/gr.6584707. PMC 2099581. PMID 18055845.
- Reik, W, Dean, W, Walter, J. (Aug 2001). "Epigenetic reprogramming in mammalian development". Science 293 (5532): 1089–93. doi:10.1126/science.1063443. PMID 11498579.
- Mancini-DiNardo, Debora; Steele, SJ; Levorse, JM; Ingram, RS; Tilghman, SM (2006). "Elongation of the Kcnq1ot1 transcript is required for genomic imprinting of neighboring genes. 2006". Genes & Development 20 (10): 1268–1282. doi:10.1101/gad.1416906. PMC 1472902. PMID 16702402.
- Metz, C. W. (1938). "Chromosome behavior, inheritance and sex determination in Sciara". American Naturalist 72 (743): 485–520. doi:10.1086/280803.
- Alleman, Mary; John Doctor (June 2000). "Genomic imprinting in plants: observations and evolutionary implications". Plant Molecular Biology 43 (2–3): 147–61. doi:10.1023/A:1006419025155. PMID 10999401.
- Tycko, B.; I. M. Morison (September 2002). "Physiological functions of imprinted genes". Journal of Cellular Physiology 192 (3): 245–58. doi:10.1002/jcp.10129. PMID 12124770.
- Constância, Miguel; Benjamin Pickard, Gavin Kelsey, and Wolf Reik (September 1998). "Imprinting mechanisms". Genome Research 8 (9): 881–900. doi:10.1101/gr.8.9.881 (inactive 2014-02-03). PMID 9750189.
- Moore, T.; D. Haig (February 1991). "Genomic imprinting in mammalian development: a parental tug-of-war". Trends in Genetics 7 (2): 45–9. doi:10.1016/0168-9525(91)90230-N. PMID 2035190. Retrieved 2008-07-01.
- Haig, D. (1997). "Parental antagonism, relatedness asymmetries, and genomic imprinting". Proceedings of the Royal Society B 264 (1388): 1657–1662. doi:10.1098/rspb.1997.0230.
- Haig, D. (2000). "The kinship theory of genomic imprinting". Annual Review of Ecology and Systematics 31 (1388): 9–32. doi:10.1146/annurev.ecolsys.31.1.9. PMC 1688715. PMID 9404029.
- McElroy, Joseph; JJ Kim, DE Harry, Brown SR, JC Dekkers, and SJ Lamont (2006). "Identification of trait loci affecting white meat percentage and other growth and carcass traits in commercial broiler chickens". Poultry Science 85 (4): 593–605. PMID 16615342.
- Tuiskula-Haavisto, M.; J. Vikki (2007). "Parent-of-origin specific QTL – a possibility towards understanding reciprocal effects in chicken and the origin of imprinting". Cytogenetic and Genome Research 117 (1–4): 305–312. doi:10.1159/000103192. PMID 17675872.
- Keverne, E; Curley, J (2008). "Epigenetics, brain evolution, and behavior". Frontiers in Neurobiology 29 (3): 398. doi:10.1016/j.yfrne.2008.03.001.
- Wolf, J. B. (2009). "Cytonuclear interactions can favor the evolution of genomic imprinting". Evolution 63 (5): 1364–1371. doi:10.1111/j.1558-5646.2009.00632.x. PMID 19425202.
- Pardo-Manuel De Villena, F; De La Casa-Esperón, E; Sapienza, C (2000). "Natural selection and the function of genome imprinting: Beyond the silenced minority". Trends in Genetics 16 (12): 573–579. doi:10.1016/S0168-9525(00)02134-X. PMID 11102708. Unknown parameter
- De La Casa-Esperón, E; Sapienza, C (2003). "Natural selection and the evolution of genome imprinting". Annu Rev Genet 37: 349–370. doi:10.1146/annurev.genet.37.110801.143741. PMID 14616065.
- Barlow, D.P. (1993). "Methylation and imprinting: from host defense to gene regulation?". Science 260 (5106): 309–310. doi:10.1126/science.8469984. PMID 8469984.
- Chai, Jing-Hua; Devin P. Locke, Tohru Ohta, John M. Greally and Robert D. Nicholls (November 2001). "Retrotransposed genes such as Frat3 in the mouse Chromosome 7C Prader-Willi syndrome region acquire the imprinted status of their insertion site". Mammalian Genome 12 (11): 813–821. doi:10.1007/s00335-001-2083-1. PMID 11845283. Retrieved 2008-07-01.
- Winstead, Edward R. (2001-05-07). "The Legacy of Solid Gold". Genome News Network.
- Yu, Yinhua; Fengji Xu, Hongqi Peng, Xianjun Fang, Shulei Zhaodagger, Yang Li, Bruce Cuevas, Wen-Lin KuoDagger, Joe W. GrayDagger, Michael Siciliano, Gordon B. Mills and Robert C. Bast Jr. (January 1999). "NOEY2 (ARHI), an imprinted putative tumor suppressor gene in ovarian and breast carcinomas". Proc. Natl. Acad. Sci. U.S.A. 96 (1): 214–9. doi:10.1073/pnas.96.1.214. PMC 15119. PMID 9874798.
- C. David Allis; Thomas Jenuwein; Danny Reinberg (2007). Epigenetics. CSHL Press. p. 440. ISBN 978-0-87969-724-2. Retrieved 10 November 2010.
- Raphaël Scharfmann (2007). Development of the Pancreas and Neonatal Diabetes. Karger Publishers. pp. 113–. ISBN 978-3-8055-8385-5. Retrieved 10 November 2010.
- Nowack, Moritz K.; Reza Shirzadi, Nico Dissmeyer, Andreas Dolf, Elmar Endl, Paul E. Grini and Arp Schnittger (May 2007). "Bypassing genomic imprinting allows seed development". Nature 447 (7142): 312–5. doi:10.1038/nature05770. PMID 17468744.
- Imprinted Gene and Parent-of-origin Effect Database
- J. Kimball's Imprinted Genes Site
- Genomic imprinting at the US National Library of Medicine Medical Subject Headings (MeSH)
- Harwell Mouse Imprinting Map