Mcl-1 expression at different stages of mammary tumorigenesis in the MMTV-PyMT mouse model resembling the progression and morphology of human breast cancers showed that tumors of mice with homozygous deletion of Mcl-1 in the mammary epithelium expressed equally high levels of Mcl-1 as those in WT tumors. The gene discussed is MCL1; the disease is breast cancer.