MMTV-PyMT mice were crossed with RosaCRE-ERT2;Mcl1fl/+ mice and cohorts aged until mammary tumours reached 5 mm diameter at which time ubiquitous deletion of one allele of Mcl1 was achieved by tamoxifen administration and tumour growth monitored until clinical endpoint (Fig. 1A). This evidence concerns the gene MAPK3 and breast cancer.