LSP1 (lymphocyte-specific protein 1) is a highly conserved phosphoprotein 1 that plays critical roles in immune cell function and migration. The protein contains two domains: an N-terminal acidic domain and a highly conserved C-terminal basic domain with putative phosphorylation sites 1. LSP1 is essential for neutrophil motility, fibrinogen matrix adhesion, and trans-endothelial migration 2. Beyond neutrophils, LSP1 marks specialized megakaryocyte subpopulations involved in inflammatory responses and pathogen recognition 3. Functionally, LSP1 negatively regulates T cell migration and Th17 cell differentiation; LSP1 deficiency increases IL-17-expressing T cells and accelerates primary Sjögren's syndrome 4. LSP1 is also a key gene associated with macrophage polarization in liver cancer, correlating positively with macrophage infiltration and representing a potential therapeutic target 5. Disease-wise, LSP1 gene variations associate with leukemia, lymphomas, and Hodgkin's disease 2. The rs3817198T>C polymorphism is significantly correlated with breast cancer risk, particularly in Caucasians and Asians, with higher LSP1 expression inversely correlating with chemotherapy drug resistance [PMID:21127985; 60].