 Biochemical analysis of cells from these mutants revealed that Lyn is essential in establishing ITIM-dependent inhibitory signaling and for activation of specific protein tyrosine phosphatases within myeloid cells.Mice that expressed a hyperactive Lyn allele were tumor free and displayed no propensity toward hematological malignancy.These mice have reduced numbers of conventional B lymphocytes, down-regulated surface immunoglobulin M and costimulatory molecules, and elevated numbers of B1a B cells.