In the present study, we isolated clones of human argininosuccinate lyase (ASL) cDNA from a liver cDNA library using a clone of rat ASL cDNA and analyzed human ASL cDNA nucleotide sequence. The results reveal that the sequence of human ASL cDNA published by O'Brien et al. in 1986 [Proc. Natl. Acad. Sci USA 83, 7211-7215] had one-base deletions at three independent positions in the coding regions near the COOH-terminus, which caused frame-shift variations in the amino acid sequence. Amino acid sequencing of peptides prepared from purified human liver ASL showed our predicted amino acid sequence to be correct.