Substrate of the antigen receptor-coupled tyrosine kinase. Plays a role in antigen receptor signaling for both clonal expansion and deletion in lymphoid cells. May also be involved in the regulation of gene expression (By similarity). Interacts (via SH2 domain) with FGR (By similarity). Associates with the SH2 and SH3 domains of LCK. Binding to he LCK SH3 domain occurs constitutively, while binding to the LCK SH2 domain occurs only upon TCR stimulation. A similar binding pattern was observed with LYN, but not with FYN in which the FYN SH2 region associates upon TCR stimulation but the FYN SH3 region does not associate regardless of TCR stimulation. Directly associates with HAX1, through binding to its C-terminal region. Interacts with HS1BP3. Interacts with FES/FPS. Forms a multiprotein complex with LYN and ANKRD54. Expressed only in tissues and cells of hematopoietic origin. Phosphorylated by LYN, FYN and FGR after cross-linking of surface IgM on B-cells. Phosphorylation by LYN, FYN and FGR requires prior phosphorylation by SYK (By similarity). Binds to LCK in vivo, and is tyrosine phosphorylated upon TCR stimulation. Phosphorylated by FES.