HCLS1 (hematopoietic cell-specific Lyn substrate 1) is a versatile actin-binding protein primarily functioning in leukocyte dynamics and immune cell signaling. As a substrate of antigen receptor-coupled tyrosine kinases, HCLS1 plays a central role in T cell receptor (TCR) and B cell receptor (BCR) signaling, regulating both clonal expansion and deletion in lymphoid cells 1. The protein orchestrates actin remodeling through Arp2/3-dependent mechanisms, enabling critical leukocyte functions including migration, adhesion, phagocytosis, and degranulation 1. HCLS1 also mediates GTPase and integrin activation downstream of multiple receptors, including CXCR4 1. Beyond immune functions, HCLS1 is upregulated during human cytomegalovirus latency in a US28-dependent manner, stabilizing actin structures and increasing monocyte motility and transendothelial migration capacity, potentially facilitating viral reservoir dissemination 2. Disease associations include primary immunodeficiencies—notably agammaglobulinemia and hyper-IgM syndrome—reflecting its essential role in lymphocyte development and function. HCLS1-associated protein X-1 (HAX1) mutations cause Kostmann disease (severe congenital neutropenia), highlighting the importance of HCLS1-interacting partners in hematopoietic homeostasis 3. Recent proteomic studies identify HAX1 as a cerebrospinal fluid biomarker for Alzheimer's disease staging 4, suggesting broader neurological relevance. HAX1 undergoes K63-linked polyubiquitination under energy stress, regulating P-body formation and protein synthesis inhibition, with implications for colorectal cancer progression 5.