C17orf99 and osteoarthritis: Furthermore, IL-40 is predominantly found in the superficial layer of cartilage in patients with osteoarthritis, where it activates chondrocytes to produce pro-inflammatory cytokines such as IL-6 and IL-8, as well as matrix-degrading enzymes including MMP-1, MMP-3, and MMP-13, thereby promoting cartilage degradation (13).