Ornithine transcarbamylase (OTC, EC 2.1.3.3), located in the mitochondriat matrix, is synthesized on membrane-free polysomes in the form of a larger precursor and is then proteolytically processed to the mature form. OTC deficiency, which is a disease with X-linked dominant inheritance, is caused by various kinds of mutation (Briand et al., 1982). We report a heterozygous patient with OTC deficiency showing a decrease of OTC protein and a truncated enzyme precursor.