Abstract Ten genomic DNA clones encoding the human leukocyte common Ag (LCA, CD45) gene were isolated by screening human genomic DNA libraries with LCA cDNA probes. One genomic DNA clone contains the promoter region and the first two exons, as determined by primer extension analyses and S1 nuclease protection studies as well as nucleotide sequence determination. The first exon does not encode a peptide, while the second exon contains the initiation ATG codon and encodes the signal peptide. The other nine genomic DNA clones, which are separated from the first genomic clone by an unknown distance, are connected and span a total of 73 kb. The nine connected genomic clones encode a total of 31 exons. The 33 exons encoded by these 10 genomic clones account for the entire cDNA sequences including the 5' and 3' untranslated sequences. Exon 3 and exons 7 through 15 encode the extracellular domain sequences that are common to all LCA isoforms. Differential usage of exons 4, 5, and 6, generates at least five distinct LCA isoforms. Exon 16 encodes the transmembrane peptide. The cytoplasmic region of the leukocyte common antigens is composed of two homologous domains. Exons 17 through 24 encode the first domain, and exons 25 through 32 encode the second domain. The comparison of these exons indicated that the homologous domains were generated by duplication of several exons. The most 3' exon (exon 33) encodes the carboxy terminus of the LCA molecules and includes the entire 3' untranslated sequence.