The gene for the mitochondrial enzyme ornithine transcarbamylase was mapped to the short arm of the X chromosome by in situ hybridization experiments, with DNA complementary to the human ornithine transcarbamylase gene used as a probe. A series of cell lines with X chromosome abnormalities was used to localize the gene to band Xp21.1. Because the gene maps near the Duchenne muscular dystrophy locus, the ornithine transcarbamylase probe may be useful in carrier detection and prenatal diagnosis of Duchenne muscular dystrophy as well as of ornithine transcarbamylase deficiency.