The leukocyte-common antigen (CD45) is a transmembrane protein tyrosine phosphatase expressed uniquely by cells of hematopoietic origin. There are multiple isoforms of CD45 that are generated by the variable use of three exons (exons 4-6). The use of the variable exons results in changes near the amino-terminus of the mature glycoprotein. The gene is located on chromosome 1 for both human and mouse in a region that is homologous between these two species. This conserved linkage group contains a number of genes of immunological interest, such as the genes for complement regulatory proteins and the FCG2 receptor. Yeast artificial chromosomes provide a vector system in which large fragments of foreign DNA can be isolated and are suited to long-range physical mapping. To this end, three yeast artificial chromosomes containing the human CD45 gene have been isolated and characterized. They overlap to span 475 kb, establishing the largest physical map for DNA within the conserved linkage group. The CD45 gene is entirely encoded within one yeast artificial chromosome clone as determined by mapping with cDNA probes. A mouse B cell line transfected with this YAC clone expressed the low-molecular-weight isoform of the protein into the cell surface. The size of the human CD45 gene was determined to be approximately 120 ± 10 kb.