HSH2D (hematopoietic SH2 domain-containing protein, also known as ALX) is an adapter protein involved in tyrosine kinase and CD28 signaling []. It is expressed exclusively in hematopoietic cells. It contains an Src homology 2 (SH2) domain, four PXXP polyproline sequences, and two likely sites of tyrosine phosphorylation. The SH2 domain plays a critical role in ALX function downstream of CD28 [].