NOTCH1 inhibition reduces mammary tumorsphere formation. (A) Quantification of tumorsphere-forming potential of primary tumor cells isolated from dissociated MMTV-tTA/TOP-ICN1 tumors left untreated or treated with doxycycline (2 μg/ml) for 8 days. Error bars denote standard error (n = 3). (B) Representative images of untreated and doxycycline-treated mammary tumorspheres showing relative difference in tumorsphere size and morphology. Examples of spheres that were counted are indicated with arrows.