Tumour cell heterogeneity affects cisplatin but not doxorubicin cytotoxicity in two phenotypes of the triple-negative breast cancer (TNBC) cell line, MDA-MB-231. A mesenchymal MDA-MB-231 phenotype was three times less sensitive to cisplatin than an epithelial-type phenotype from the same cell line, which has important implications in the success of TNBC chemotherapies.