Although Notch1 has been previously shown to play a role in the expansion of premalignant and tumor-initiating cells in NICD-induced mammary tumors [55,57], we did not find any differences in the primary or secondary tumorsphere forming ability of primary tumors derived from Postn-null mice (Figure S6 in Additional file 7), suggesting that the loss of Postn has no effect on the primary tumor stem cell compartment. The gene discussed is NOTCH1; the disease is breast cancer.