CSN1S1 (casein alpha s1) is a multifunctional milk-derived protein with roles extending beyond nutrition into immunomodulation and disease pathogenesis. Primary function: CSN1S1 acts as an opioid antagonist with vasorelaxing activity [UniProt], though its predominant characterized role is immunomodulatory. Mechanism: CSN1S1 induces proinflammatory cytokine expression (IL-1β, IL-6, IL-8) in monocytic cells primarily through TLR4 receptor signaling and the inflammasome pathway 1, with effects inhibited by casein kinase 2 phosphorylation 1. CSN1S1 skews monocyte differentiation toward macrophages via ERK1/2 activation 2. In mammary tissue, CSN1S1 negatively regulates β-casein synthesis by inhibiting JAK2/STAT5a signaling 3. Disease relevance: CSN1S1 shows reduced expression and elevated promoter methylation in breast cancer tissues, correlating positively with survival 4. CSN1S1 variants act as recessive modifiers delaying Alzheimer's disease age-of-onset by ~6 years in PSEN1 mutation carriers 5. CSN1S1 is elevated in osteoarthritic meniscus 6 and identified as a novel biomarker in epithelial ovarian cancer 7. Clinical significance: CSN1S1 represents a potential therapeutic target and surveillance biomarker for breast cancer 4 and may inform understanding of milk's role in immune development.