CDKN2A and breast cancer: Both genetic alterations are not mutually exclusive in claudin-low mammary cancer as demonstrated in the EF1-tTA-based overexpression model of oncogenic KRAS, which corresponds closely to the mutation status or lack of expression of CDKN2A and TRP53 in four of the seven human claudin-low breast cancer cell lines, including MDA-MB-231, SUM159PT, and Hs578T that carry mutant RAS.