Lyn is a Src-family protein tyrosine kinase that is predominantly expressed in hematopoietic cells. It is associated with a number of cell surface receptors including the B cell antigen receptor (BCR) and Fc receptors. Upon their triggering, Lyn phosphorylates subunits of these receptors in a cholesterol-dependent manner, utilizing the plasma membrane lipid raft system. The phosphorylated intracellular domains of the receptors are accessible for cytoplasmic Syk tyrosine kinase, which is activated by Lyn-mediated phosphorylation and which transduces the signal to downstream adaptors. Lyn is abnormally distributed in acute myeloid leukemia cells and seems to be a novel pharmacologic target of this disease.