Lyn is a 56 kd tyrosine kinase that is similar to mouse T-lymphocyte-specific tyrosine kinase p56lck and the v-yes protein as well as to the gene products of v-fgr and v-src. Northern hybridization analysis showed that a 3.2-kilobase Lyn mRNA was expressed in a variety of tissues of the human fetus (1). Lyn is expressed preferentially in B cells and can be coimmunoprecipitated with IgM suggesting that Lyn is physically associated with membrane-bound IgM, and participates in antigen-mediated signal transduction (2). Crosslinking of membrane-bound IgM with antibody induces rapid increase in activities of Lyn and Lyn-associated phosphatidylinositol 3-kinase (3). Crosslinking of B-cell antigen receptor also induces association of Lyn with an 85