The intracellular tyrosine kinase PTK6 (also called BRK for Breast tumor kinase) is an intracellular tyrosine kinase that is evolutionarily related to SRC family kinases.6 PTK6 is overexpressed in a majority of human breast cancers and in most breast tumor cell lines.7, 8, 9, 10 Previously, we detected PTK6 expression in a normal mammary gland human breast tissue array, as well as in a human breast tumor array using immunohistochemistry. Here, PTK6 is linked to breast cancer.