Growth factor which acts through erb b-4 rtk

A method of regulating an endogenous protein activity affecting ErbB-4 activity in a cell, the method comprising the steps of contacting the cell with an agent for regulating the endogenous protein activity, the endogenous protein being at least 95% homologous to positions 4-50 of SEQ ID NO:

• 15 as determined using the BestFit software of the Wisconsin sequence analysis package, utilizing the Smith and Waterman algorithm, where gap creation penalty equals 8 and gap extension penalty equals 15, said polypeptide being capable of binding ErbB-4.