Human CD137 (4-1BB; TNFRS9) is an inducible costimulatory molecule that activates T cells. CD137-CD137L mediated signaling has been shown to be important for proliferation, effector functions and survival of T cells. CD137 is also expressed in NK and NKT cells. Antibodies targeting CD137 activation in immune cells have demonstrated potent anti-tumor effects in cancer patients.