A novel CD45 transcript involving the alternative splicing of a new exon is widely distributed in lymphoid tissues.

Protein tyrosine phosphatase CD45 is a key player in T-cell receptor signaling and lymphocyte development. Differential expression of multiple CD45 isoforms resulting from the alternative splicing of exons A, B, and C, which encode part of the extracellular domain, is an important feature of CD45 expression. We report a novel isoform that results from the alternative splicing of a previously undiscovered exon between the constitutively spliced exon 3 and the alternatively spliced exon A. This 123-bpâ€:#x0093:long exon encodes 41 amino acids and is unlikely to undergo te extensive glycosylation seen for the regions encoded by exons A, B, and C. Reverse transcriptase polymerase chain reaction demonstrates that this isoform is expressed in human peripheral blood mononuclear cells and cell lines of lymphoid origin, but with a clearly different pattern to that of the isoforms caused by exons A, B, and C, implying a different regulatory mechanism.