Crk, is a transformation-specific protein that induces elevation of cellular phosphotyrosine levels, but lacks tyrosine kinase activity itself. A second protein, Nck, consists solely of three SH3 domains and one SH2 domain, while GRB2 contains an SH2 domain flanked on both sides by SH3 domains. A member of this protein class, Crk-L, is encoded by a gene located on chromosome 22, band 11, centromeric of the chronic myelogenous leukemia breakpoint region. Crk-L encodes a 303 amino acid protein with one SH2 and two SH3 domains, and a predicted molecular weight of 36 kDa.