Involved in T-cell antigen receptor mediated signaling. Interacts with SLA. Interacts with CBLB (By similarity). Interacts with GRB2 (PubMed:7706237). Interacts with SHB (PubMed:12084069). Interacts with PRAM1 (PubMed:11301322). Interacts (via SH2 domain) with CD6 (via tyrosine phosphorylated C-terminus) (PubMed:16914752, PubMed:24584089). Interacts with FYB1 and the phosphorylated form of FYB2 (PubMed:27335501). Highly expressed in spleen, thymus and peripheral blood leukocytes. Highly expressed also in T-cell and monocytic cell lines, expressed at lower level in B-cell lines. Not detected in fibroblast or neuroblastoma cell lines. The SH2 domain mediates interaction with phosphorylated CD6 (PubMed:16914752). The SH2 domain mediates interaction with SHB (PubMed:12084069). 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. The disease is caused by variants affecting the gene represented in this entry.