A full-length cDNA clone encoding an isoform of human CTP synthetase (type II) was isolated. A 1761-nucleotide open reading frame which corresponds to a protein of 586 amino acids with a predicted molecular mass of 65678 Da was identified. The predicted protein sequence showed 74% identity with the translation product of a previously identified human CTP synthetase cDNA clone (type I). The function of the human cDNA encoding type II CTP synthetase was verified by successful complementation of the cytidine-requiring CTP synthetase deficient mutant JF618 of Escherichia coli. The gene encoding type II CTP synthetase has been localized on chromosome Xp22.