Modeling studies show that increased expression of nerve growth factor is associated with increased pain. This study tested the safety and efficacy of the monoclonal antibody tanezumab — which targets nerve growth factor — for the treatment of knee pain in patients with osteoarthritis. Nerve growth factor is a neurotrophin that regulates the structure and function of responsive sensory neurons, including small-diameter nociceptive afferents. There has been increasing recognition of the potential role of nerve growth factor in pain modulation through nociceptor sensitization. 1 – 7 In animals and humans, exogenous nerve growth factor increases pain either locally or systemically, depending on the dose and the route of administration. 8 , 9 Increased expression of nerve growth factor is found in inflamed tissues from patients with conditions such as arthritis, pancreatitis, and prostatitis. 10 – 12 Levels of nerve growth factor are also elevated in animal models of inflammatory pain, . . .