Tumor necrosis factor receptor superfamily member 9(TNFRSF9) is an inducible T cell surface protein belonging to the TNF receptor superfamily. It is a single-pass type I membrane protein which contains 4 TNFR-Cys repeats. The human and mouse proteins share 60% amino acid sequence identity. It is absent from naive T cells, but upregulated and continually expressed following T cell activation. It is a receptor for TNFSF9/4-1BBL, and possibly active during T cell activation.