The retarded hair growth () mutation in mice is an allele of ornithine aminotransferase ().

Because of the similar phenotypes they generate and their proximate reported locations on Chromosome 7, we tested the recessive retarded hair growth () and frizzy () mouse mutations for allelism, but found instead that these defects complement. To discover the molecular basis of , we analyzed a large intraspecific backcross panel that segregated for and restricted this locus to a 0.9 Mb region that includes fewer than ten genes, only five of which have been reported to be expressed in skin. Complementation testing between and a recessive null allele of fibroblast growth factor receptor 2 eliminated as the possible basis of the retarded hair growth phenotype, but DNA sequencing of another of these candidates, ornithine aminotransferase (), revealed a G to C transversion specifically associated with the allele that would result in a glycine to alanine substitution at residue 353 of the gene product. To test whether this missense mutation might cause the mutant phenotype, we crossed mice with mice that carried a recessive, perinatal-lethal, null mutation in Oat (designated herein). Hybrid offspring that inherited both and displayed markedly delayed postnatal growth and hair development, indicating that these two mutations are allelic, and suggesting strongly that the G to C mutation in is responsible for the retarded hair growth phenotype. Comparisons among +/+, /+, and mice showed plasma ornithine levels and ornithine aminotransferase activities (in liver lysates) consistent with this assignment. Because histology of 7- and 12-month-old and retinas revealed chorioretinal degeneration similar to that described previously for / mice, we suggest that the mutant may offer an ideal model for gyrate atrophy of the choroid and retina (GACR) in humans, which is also caused by the substitution of glycine 353 in some families.