Hormonal control of the growth and cellular function of both normal and neoplastic tissue first requires binding of the hormone to the target tissue cells. This binding, which determines the cellular responsiveness to the hormone, occurs at sites on or within the cells. Each hormone has high affinity, specific receptors, or binding sites to which it can bind to initiate its unique effect. Furthermore, the cellular effect of a hormone depends both on the form and levels of the circulating hormone and the responsiveness of the target tissue cell to that hormone. If a character of the receptor is altered, it is possible that its ability to interact with the hormone and, thus, the cellular function, may also be altered.