Description: This foundational structure of human Oncostatin M—a cytokine that inhibits tumor cell growth while promoting inflammatory responses—reveals the four-helix bundle topology and maps the site 2 (gp130-binding) and site 3 (LIFR/OSMR-binding) receptor epitopes to atomic resolution. The structure-function dissection provides key insights into cytokine-receptor specificity within the gp130 signaling family, with implications for cancer and inflammatory disease biology.