RB1 and cancer: Few DNA damage response antibodies were available and so selection was limited to well-validated proteins associated with cancer, such as p53, Rb, EGFR etc. As p63 is essential for the proliferative potential of stem cells in stratified epithelia [32], we postulated that p63+ cells would express higher levels of the epithelial marker protein E-cadherin, compared with p63− cells and this was confirmed by the protein array (p = <0.0001) (Figure 3A).