NOTCH1 and neoplasm: To investigate the role of NOTCH1 in mammary gland development and tumor progression, we mated mice expressing the human intracellular form of NOTCH1 under control of a tet-responsive promoter (Tet-Op-ICN1) with mice expressing the tet Transactivator under control of the MMTV promoter (MMTV-tTA) [34,41], allowing us to modulate expression of intracellular NOTCH1 in the developing mouse mammary gland.