TGFB1 and breast cancer: We examined the rapamycin sensitivity of matched cell lines derived from MMTV-PyMT mouse mammary tumors [18] with a functional TGFβ signaling pathway or that lacked TGFβ responsiveness due to Cre Recombinase-mediated deletion of the floxed gene encoding the TGFβ type II receptor (TβRII) [19].