Hematopoietic lineage cell-specific protein (HCLS1) is a protein that in humans is encoded by the HCLS1 gene. It is expressed only in tissues and cells of hematopoietic origin. It is substrate of the antigen receptor-coupled tyrosine kinase and plays a role in antigen receptor signaling for both clonal expansion and deletion in lymphoid cells. It may also be involved in the regulation of gene expression.