HS1 is expressed in lymphoid and hematopoietic cells, and is heavily post-translationally modified. HS1 deficient mouse models have demonstrated the protein's role in receptor-mediated apoptosis and proliferation. HS1 is phosphorylated at Tyr378 and Tyr397 by the kinase Syk, providing a high-affinity binding site for SH2 domains from the Src family. Following this interaction, HS1 is then phosphorylated at Tyr222 by c-Fgr, Lyn, and Fyn kinases. HS1 plays an important role in T cell signaling, where HS1 phosphorylation recruits and activates Vav1 at the immune synapse. As a homolog of the actin binding protein cortactin, HS1 has been shown to mediate neutrophil chemotaxis through phosphorylation of tyrosines 222, 378, and 397.