Neuregulin (heregulin) polypeptides have been shown to interact with and activate the ErbB2, ErbB3 and ErbB4 receptor proteins, and have been found to be essential for the normal development of the cardiovascular, pancreatic and nervous systems. Neuregulins are involved in the normal development of mammary gland, are secreted by breast cancer cells, and can alternatively promote or inhibit breast cancer cell division. [ ]