Since B9/B9L double-homozygous mutants of HD1 were lethal [13], we crossed heterozygous Bcl9/Bcl9LΔHD1/fl mutant mice with MMTV-PyMT transgenic mice to induce mammary tumor formation and with MMTV-Cre mice to delete the floxed allele in mammary tumors of Bcl9/Bcl9LΔHD1/fl mutant mice. The gene discussed is BCL9; the disease is breast cancer.