BCL9 and breast cancer: Since B9/B9L double-homozygous mutants of HD2 were lethal [13], we crossed heterozygous Bcl9/Bcl9LΔHD2/fl mutant mice with MMTV-PyMT transgenic mice to induce mammary tumor formation and with MMTV-Cre mice to delete the floxed alleles in mammary tumors of Bcl9/Bcl9LΔHD2/fl mutant mice (Fig. 5A).