This mutation update addendum summarizes 30 new mutations and polymorphisms found in the ornithine transcarbamylase (OTC) gene since the publication in this journal of the first mutation update. Thus, more than 60 mutations and polymorphisms in the OTC gene are currently known. Most of the mutations have been seen in a single family and the few recurrent mutations occurred in CpG dinucleotides. The presumed deleterious effects of most mutations await confirmation by appropriate expression studies. Once the tertiary structure of the enzyme is fully known, and the functional domains established, the effects of mutations, or lack thereof, could be better predicted.