CSN3 (casein kappa) has dual functions depending on context. In milk physiology, kappa-casein stabilizes micelle formation and prevents casein precipitation, playing a critical role in milk protein composition 1. CSN3 genetic variants significantly influence milk protein profiles in dairy cattle, with the B allele associated with increased kappa-casein expression 1. The gene exhibits substantial genetic variation across species, with multiple polymorphisms identified in horses, goats, and camels that may alter protein function 234. Beyond lactation, CSN3 encodes the third subunit of the COP9 signalosome (CSN), an eight-subunit protein complex functioning as a deneddylase and regulator of cullin-RING ubiquitin ligases 5. In cancer biology, CSN3 promotes oncogenic pathways: in osteosarcoma, CSN3 drives cisplatin resistance through FOXO3-mediated autophagy induction 6, and in hepatocellular carcinoma, CSN3 knockdown triggers G0/G1 cell cycle arrest and apoptosis 7. Additionally, CSN3 selectively inhibits TNF-induced NF-κB activation by interacting with IKKgamma, demonstrating immunoregulatory function 8. These findings suggest CSN3 has evolved distinct roles in lactation and cellular signaling, with therapeutic potential as a target for overcoming chemoresistance in cancer.