SAMD5 (sterile alpha motif domain containing 5) is a cytoplasmic signaling regulator with context-dependent roles in cell proliferation and disease pathology. In breast cancer, SAMD5 functions as a tumor suppressor, with overexpression significantly reducing triple-negative breast cancer (TNBC) cell viability, colony formation, and invasion by downregulating the c-Myc signaling pathway and its effectors (β-catenin, CDK4, CDK6, Cyclin D1) 1. This suppressive activity occurs through negative regulation of Polo-like Kinase 1 (PLK1), as PLK1 overexpression counteracts SAMD5's inhibitory effects 1. In vivo xenograft studies confirmed that SAMD5 overexpression reduced tumor weight and volume 1. Conversely, in prostate cancer, SAMD5 mRNA is upregulated and independently predicts biochemical recurrence after radical prostatectomy, indicating oncogenic properties in this context 2. SAMD5 also shows disease association relevance: an intronic variant (rs844586) associates with intrathecal IgG synthesis in multiple sclerosis, independent of MHC effects 3, while a male-specific locus near SAMD5 shows protective association with renal cell carcinoma 4. Additionally, SAMD5 genetic variants on chromosome 6 contribute to aerobic fitness responses to high-intensity interval training, potentially via angiogenic pathways 5. SAMD5 expression patterns vary by tissue context, with nuclear localization in cholangiocarcinoma but cytoplasmic expression in normal bile ducts 6.