A cDNA clone of the argininosuccinate lyase gene (ASL) was isolated from an adult human liver library by probing with synthetic oligonucleotide probes. This clone and a yeast genomic DNA fragment containing the ASL gene were sequenced using the M13-dideoxynucleotide method. Comparison of the yeast and human clones at the nucleotide and putative amino acid sequence levels indicated identities of 50 and 54%, respectively. The most conserved region of the yeast gene was used to detect human clones in the liver cDNA library to test phylogenetic screening capabilities of conserved genes. ASL was mapped to human chromosome 7pter----q22 using human-mouse somatic cell hybrid DNA and further mapped by in situ hybridization to chromosome 7cen----q11.2 on human metaphase chromosomes. The probe also detected a sequence on chromosome 22. Somatic cell hybrid DNA digested with PvuII revealed a mouse polymorphism between Balb/c and C3H mice in the ASL gene.