Growth factors are polypeptides that interact with specific cellular receptors leading to many different biological responses. There are various families of growth factors that have similar biochemical structures. Although many growth factors stimulate cell proliferation, a few have primarily inhibitory functions, such as transforming growth factor-p (TGFB). Growth factors regulate various modes of action of endocrine tissues, including autocrine, paracrine, and endocrine functions. Recent studies have shown that growth factors also regulate various proteins in the cell cycle, and may have a direct or indirect effect on cell proliferation. For example, TGFB regulates various inhibitory cell-cycle proteins, including p27(kip1) and p15(INK4B). Molecular analyses of growth factors, including the cloning and sequencing of specific growth factor receptors, have contributed greatly to our understanding of the role these factors play in cellular homeostasis and neoplastic development.