Ornithine transcarbamylase (OTC) deficiency (hyperamonaemia type II; MIM 311250) is an X-linked disease inherited by a partially dominant trait. Clinical manifestations are more severe in hemizygous males than in heterozygous females. To date about 160 different mutations and 10 polymorphisms have been identified in the OTC gene. The mutations are mostly point mutations and are equally distributed within the gene. The majority of the OTC gene mutations occurred in the paternal germ cells. Treatment of the patients requires restriction of dietary protein intake and activation of other pathways of waste nitrogen. Recently, a study of gene therapy has been started. It consists of the transfer of cloned ornithine transcarbamylase sequence into cultured cells and animals.