Breast tumor kinase, Brk (also known as PTK6), is a member of intracellular tyrosine kinase family that is thoμght to be related to Src family of tumor related kinases. Brk is expressed in breast carcinomas but not in normal breast tissue. The presence of Brk in the nucleus appears to be linked to suppression of tumor progression and has been shown to undergo autophosphorylation.