Deficiency of argininosuccinate synthetase causes arginine auxotrophy in lower organisms and causes citrullinemia in humans and cattle. Previously, seven missense mutations, four mutations associated with an absence of an exon in mRNA, and one splicing mutation have been identified in human neonatal citrullinemia. Reverse transcription of mRNA, amplification of cDNA and sequencing of cDNA clones were used to identify two additional missense mutations causing citrullinemia. One mutation involves substitution of leucine for serine at position 18 (S18L) and the other a substitution of cysteine for arginine at position 86 (R86C). Both of these mutations represent C----T transitions in CpG dinucleotides, and eight of nine missense mutations causing human citrullinemia involve similar transitions in CpG dinucleotides. The nucleotide coding sequence and deduced amino acid analysis are available for four mammalian species, yeast and three bacterial species. Six of nine missense mutations in humans occur in amino acid positions that are completely conserved in these organisms. Mutations causing human citrullinemia are extremely heterogeneous, and all non-consanguineous individuals studied to date are compound heterozygotes.