Nerve growth factor (NGF) is a polypeptide involved in the regulation of growth and differentiation of sympathetic and certain sensory neurons. NGF consists of 3 types of subunits, alpha, beta and gamma, which specifically interact to form a 7S, approximately 130,000-molecular weight complex. It is mainly expressed in Sertoli and interstitial cells. The proforms of NGF are secreted and cleaved extracellularly by the serine protease plasmin and by selective matrix metalloproteinases. NGF is important for the growth, maintenance, and survival of certain target neurons (nerve cells). It also functions as a signaling molecule.