PTK6 and melanoma: [3] PTK6 promotes cell differentiation in normal epithelial cells and is barely expressed in mature epithelial cells in gastrointestinal tract, breast or skin. [3]–[5] Although aberrant overexpression of PTK6 has been identified in several epithelial cancers, including cancers of the breast, lung, melanoma, prostate, colon, and ovary, the functions of PTK6 in cancer biology have not been fully characterized. [3], [6]–[12].