Our results indicating that Notch1-expressing cells are extremely sensitive to hormonal stimulation and require estrogen to proliferate may partly explain why ectopic Notch activation in the mammary gland results in mammary tumors only upon pregnancy, suggesting that the cells we have characterized in this study may be linked to the increase in human breast cancer found to be associated with pregnancy and may be at the origin of hormonal-dependent human breast cancer subtypes. The gene discussed is NOTCH1; the disease is breast cancer.