The MMTV-PyMT mouse recapitulates features of human breast cancer progressing through hyperplasia to metastasis36 in which we find high levels of MCL-1 in primary and metastatic lesions (Fig. 5a) and are able to genetically manipulate Mcl1. We tested the impact of reduction (heterozygous loss, HET) or deletion (homozygous loss, HOM) of MCL-1 on tumour development and metastatic spread in this model by utilising MMTV-Cre to drive specific deletion of Mcl1fl/fl in the mammary epithelium of female MMTV-PyMT mice (Supplementary Fig. S4A). Here, MCL1 is linked to breast cancer.