Abstract : Cancer results from a breakdown in the normal mechanisms which control the growth and division of cells. For many cells, the signals for initiating cell division are hormone molecules that bind to receptors on the surface of the cell. The signal is transmitted through the cell membrane by one or more receptor molecules. Receptors in an "active" state (with hormones bound) initiate a cascade of events which starts the replication machinery. Uncontrolled cell transformation often results when the receptors on the cell surface are "turned on" even in the absence of the signaling hormone. The research undertaken here has been to study the structure of one critical membrane receptor, the neu or erbB-2 receptor, whose activation has been associated with a large number of breast and ovarian cancers.