HS1, also called hematopoietic cell-specific LYN substrate 1 (HCLS1), LckBP1, p75, and hematopoietic lineage cell-specific protein, is expressed only in tissues and cells of hematopoietic origin (1,2). HS1 contains 4 cortactin repeats and one SH3 domain (2). It is phosphorylated after activation of immune receptors (3-5), leading to its recruitment to the immune synapse. Phosphorylation of HS1 is also required for regulating actin dynamics and provides docking sites for many other signaling molecules, such as Vav1 and PLCg1 (6). HS1 also plays an important role in platelet activation (7).