We derived cell lines from a PTEN‐ and TP53‐negative, EGFR‐positive primary mammary tumor that developed in a Wap‐cre:Ptenfl/fl:Tp53fl/fl mouse and that was histologically classified as a carcinosarcoma resembling a spindle‐cell, triple‐negative type of tumor that can be found in the human breast (Fig EV2B). This evidence concerns the gene TP53 and carcinosarcoma.