SPN (sialophorin, also known as SPINOPHILIN/PPP1R9B) is a multifunctional protein serving as a regulatory subunit of protein phosphatase 1 (PP1) 1. As a tumor suppressor, SPN regulates protein-protein interactions in cell signaling pathways and dephosphorylates retinoblastoma protein (pRB) during cell cycle progression 1. Low SPN levels correlate with higher tumor grade and worse prognosis across multiple human malignancies, while SPN mutations, including the oncogenic A566V variant, promote p53-dependent tumorigenesis by expanding the cancer stem cell pool in breast tumors 1. Gene Ontology annotations indicate SPN functions in immune regulation, including positive regulation of T cell migration and TNF production, and negative regulation of T cell proliferation, consistent with roles in lymphocyte activation and immune response coordination [GO:0005615]. The protein localizes to cell surface structures including microvilli and uropods, positioning it as a transmembrane signaling receptor involved in leukocyte tethering and rolling [GO:0005615]. UniProt data suggest SPN protects cells from apoptosis, promoting survival through anti-apoptotic mechanisms. Together, these functions implicate SPN in both tumor suppression and immune cell regulation, with clinical relevance for cancer prognosis and immunotherapy.