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). The gene discussed is MCL1; the disease is breast cancer.