CD45 (PTPRC) is a family of high-molecular weight glycoproteins uniquely expressed on the surface of all leukocytes and their hematopoietic progenitors. The CD45 family consists of multiple members that are all products of a single gene. Structurally, CD45 is composed of one fibronectin extracellular domain, one transmembrane domain, and two cytoplasmic tyrosine phosphatase domains. It is essential for B- and T-cell activation. For both cell-types, CD45 has been shown to interact with kinases including Lyn, src family kinase Lck, and Janus-family kinases. In T cells, CD45 is required for the initiation of receptor signaling by dephosphorylating a negative regulatory tyrosine in the C-terminal tail of Lck. CD45 may also negatively regulate Lck by dephosphorylating the tyrosine in the activation loop, thereby attenuating Lck activity.