TP53 and breast neoplasm: To examine if this induced replication stress and a p53‐dependent cell cycle arrest following drug washout, we generated p53‐KO MCF7s by CRISPR/Cas9 gene editing (Appendix Fig S5) and compared these to a well‐established p53‐null HR+ breast tumour line, T47D, both of which could also arrest efficiently in 1 μM palbociclib for up to 7 days (Fig EV4B and C).