Heterozygous females with OTC deficiency, where one X chromosome carries the healthy OTC allele and the other X chromosome carries the mutant allele, have livers containing a mixed population of hepatocytes with normal or deficient/absent OTC enzymatic activity, depending on which X chromosome is inactivated in any given hepatocyte.32 The gene discussed is OTC; the disease is ornithine carbamoyltransferase deficiency.