Involved in T-cell antigen receptor mediated signaling. Interacts with SHB. Interacts with PRAM1 (By similarity). Interacts with SLA (PubMed:10662792). Interacts with GRB2 (PubMed:7706237). Interacts with CBLB (PubMed:10646608). Interacts (via SH2 domain) with CD6 (via tyrosine phosphorylated C-terminus) (PubMed:16914752, PubMed:24584089). Interacts with FYB1 and the phosphorylated form of FYB2 (By similarity). Highly expressed in spleen, thymus, and peripheral blood leukocytes. The SH2 domain mediates interaction with SHB. Phosphorylated after T-cell receptor activation by ZAP70, ITK and TXK, which leads to the up-regulation of Th1 preferred cytokine IL-2. SYK-dependent phosphorylation is required for recruitment of PI3K signaling components (By similarity).