ERBB2 and neoplasm: In light of the very high TIC frequency in the mouse mammary tumors we studied, the unavailability of markers that might facilitate fractionation of TIC from non-tumorigenic cells in the MMTV-Neu model [19] and the finding that FACS compromises TIC survival [43], we did not attempt to sort the bulk tumor cell populations of the models we studied into tumorigenic and non-tumorigenic fractions.