|Classification and external resources|
Cartilage–hair hypoplasia (CHH), also known as McKusick type metaphyseal chondrodysplasia,:578 is a rare genetic disorder. It is a highly pleiotropic disorder that clinically manifests by form of short-limbed dwarfism due to skeletal dysplasia, variable level of immunodeficiency and predisposition to malignancies in some cases. It was first reported in 1965 by McKusick et al. Actor Verne Troyer is affected with this form of dwarfism, as was actor Billy Barty, who was renowned for saying "The name of my condition is Cartilage Hair Syndrome Hypoplasia, but you can just call me Billy."
CHH is an autosomal recessive inherited disorder. A rarely encountered genetic phenomenon, known as uniparental disomy (a genetic circumstance where a child inherits two copies of a chromosome from one parent, as opposed to one copy from each parent) has also been observed with the disorder.
An association between mutations near or within the ncRNA component of RNase MRP, RMRP, has been identified. The endoribonuclease RNase MRP is a complex of RNA molecule and several proteins and it participates on cleavage of mitochondrial primers responsible for DNA replication and on pre-rRNA processing in the nucleolus. The locus of the gene has been mapped to the short arm of chromosome 9.
Patients with CHH usually suffer from cellular immunodeficiency. In the study of 108 Finnish patients with CHH there was detected mild to moderate form of lymphopenia, decreased delayed type of hypersensitivity and impaired responses to phytohaemagglutinin. This leads to susceptibility to and, in some more severe cases, mortality from infections early in childhood. There has also been detected combined immunodeficiency in some patients  Patients with CHH often have increased predispositions to malignancies.
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