Having shown that acute deletion of just one allele of Mcl1 impedes the growth of established tumours in vivo we investigated the impact of targeting both alleles of Mcl1. To circumvent lethality due to the requirement for MCL-1 in haematopoietic stem cells and cardiomyocytes, we used a tumour fragment transplantation system where MMTV-PyMT tumours harbouring either WT or floxed alleles of Mcl1 are transplanted into syngeneic recipients prior to tamoxifen-activated CRE-ERT2 recombination (Fig. 1D) [22, 23, 30]. This evidence concerns the gene MCL1 and neoplasm.