Trithorax Group (TrxG) proteins are a heterogeneous collection of proteins whose main action is to maintain gene expression. They can be categorized into 3 general classes based on molecular function:
plus other TrxG proteins not categorized in the first three classes.
The founding member of TrxG Proteins, trithorax (trx), was discovered by P.W. Ingham as part of his doctoral thesis while a graduate student in the laboratory of J.R.S. Whittle at the University of Sussex. Myeloid/lymphoid or mixed-lineage Leukemia (MLL) is the human homolog of trx.
|absent, small or homeotic discs 1||ash1|
|absent, small or homeotic discs 2||ash2|
|Brahma associated protein 55kD||Bap55|
|Brahma associated protein 60 kD||Bap60|
|Enhancer of bithorax||E(bx)|
|enhancer of yellow 3||SAYP or e(y)3|
|eyelid||eld or osa|
|female sterile (1) homeotic||fs(1)h|
|little imaginal discs||lid|
|modifier of mdg 4||mod(mdg4), E(var)3-93D, or doom|
|Nucleosome remodeling factor-38kD||Nurf38|
|Ubiquitously transcribed tetratricopeptide repeat, X chromosome||Utx|
The table contains names of Drosophila TrxG members. Homologs in other species may have different names.
Trithorax Group (TrxG) proteins typically function in large complexes formed with other proteins. The complexes formed by TrxG proteins are divided into 2 groups: Histone-modifiying complexes and ATP-dependent chromatin-remodeling complexes. The main function of TrxG proteins, along with polycomb group (PcG) proteins, is regulating gene expression. Whereas, PcG proteins are typically associated with gene silencing, TrxG proteins are most commonly linked to gene activation. The Trithorax complex activates gene transcription by inducing trimethylation of lysine 4 of histone H3 (H3K4me3) at specific sites in chromatin recognized by the complex. This gene activation is reinforced by acetylation of histone H4. The actions of TrxG proteins are often described as 'antagonistic' of PcG proteins function. Aside from gene regulation, evidence suggests TrxG proteins are also involved in other processes including apoptosis, cancer, and stress responses.
Role in development 
During development, TrxG proteins maintain activation of required genes, particularly the Hox genes, after maternal factors are depleted. This is accomplished by preserving the epigenetic marks, specifically H3K4me3, established by maternally-supplied factors. TrxG proteins are also implicated in X-chromosome inactivation, which occurs during early embryogenesis. Currently, it is unclear whether TrxG activity is required in every cell during the entire development of an organism or only during certain stages in certain cell types.
See also 
- Histone-Modifying Enzymes
- Histone deacetylases
- PRMT4 pathway
- Histone methyltransferase
- Histone acetyltransferase
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- Brock HW, Fisher CL (March 2005). "Maintenance of gene expression patterns". Dev. Dyn. 232 (3): 633–55. doi:10.1002/dvdy.20298. PMID 15704101.
- Soshnikova N (August 2011). "Dynamics of Polycomb and Trithorax activities during development". Birth Defects Res. Part A Clin. Mol. Teratol. 91 (8): 781–7. doi:10.1002/bdra.20774. PMID 21290568.
- Pullirsch D, Härtel R, Kishimoto H, Leeb M, Steiner G, Wutz A (March 2010). "The Trithorax group protein Ash2l and Saf-A are recruited to the inactive X chromosome at the onset of stable X inactivation". Development 137 (6): 935–43. doi:10.1242/dev.035956. PMC 2834461. PMID 20150277.
- Schuettengruber B, Martinez AM, Iovino N, Cavalli G (December 2011). "Trithorax group proteins: switching genes on and keeping them active". Nat. Rev. Mol. Cell Biol. 12 (12): 799–814. doi:10.1038/nrm3230. PMID 22108599.
- The Polycomb and Trithorax page of the Cavalli lab This page contains useful information on Polycomb and trithorax proteins, in the form of an introduction, links to published reviews, list of Polycomb and trithorax proteins, illustrative power point slides and a link to a genome browser showing the genome-wide distribution of these proteins in Drosophila melanogaster.