Lyn is a tyrosine protein kinase and a member of the src subfamily. Two isoforms of Lyn have been reported: Lyn A, 53 kD and Lyn B, 56 kD. Lyn is predominantly expressed in macrophages, B cells, and platelets. Lyn exhibits both stimulatory activity (mast cells) and inhibitory activity (B cells), and has been shown to increase p53 levels and inhibit nuclear export. Recent reports have suggested that Lyn may participate in the induction of apoptosis by genotoxic agents. Lyn has been shown to be activated in late G1 by stem cell factor and is increased in myeloid leukemias. Lyn interacts with the high affinity IgE receptor, actin cytoskeleton, gamma-tubulin, and p53. The LYN-01 monoclonal antibody recognizes the human, mouse, and rat Lyn and has been shown to be useful for Western blotting, immunoprecipitation, and immunocytochemistry.