Cartilage-hair hypoplasia (CHH) is a pleiotropic disorder characterized by short-limbed dwarfism, coneshaped epiphyses, metaphyseal flaring and irregularities, sparse and light colored hair of abnormally small caliber, immunological defects manifested by unusual susceptibility to chickenpox, and other hematological anomalies such as neutropenia and anemia. Some patients may also have Hirschsprung disease, impaired spermatogenesis, and increased susceptibility to cancer. Skeletal abnormalities may include incomplete extension at the elbows, chest deformities, lumbar lordosis, joint laxity, bowed legs, genu varum, and excessively long fibula distally relative to the tibia, in addition to dwarfism. Hypoplasia of the cartilage is seen microscopically. Metaphyseal dysplasia without hypotrichosis was initially described as a distinct entity from CHH with identical skeletal features, and lacking hypotrichosis and immunodeficiency. Anauxetic dysplasia is a form of extreme short stature of prenatal onset, hypodontia, and mild mental retardation. Recent data shows that these three disorders are allelic as each condition is caused by mutations in the RNase MRP (RMRP) gene. The RMRP gene encodes the RNA subunit of a RNA processing enzyme complex (endoribonuclease).