Notch pathway, an evolutionarily conserved signaling, controls development, differentiation and proliferation. Notch1 and Notch4 activation caused mammary tumor formation showing an oncogenic effect. Overexpression of Notch1 can also supress proliferation in breast epithelial cells depending on dosage and cell type. Thus, Notch can act as an oncogene or tumor suppressor in breast. IRF6, a member of interferon regulatory factor family, has a role in development and differentiation of the epidermis, downstream of Notch signaling. IRF6 overexpression induces cell cycle arrest in breast cancer cells showing a tumor suppressor role. It was recently identified that IRF6 is a mediator of Notch in proliferation and transformation of breast epithelial cells. In this study, it was aimed to identify whether IRF6 has any effect on cell cycle regulation, apoptosis and breast cancer stem cell population (BCSCs) under Notch and whether IRF6 has a role in expression of luminal and basal markers in breast cell lines. Our results showed that IRF6 knockdown in normal breast epithelial cell line, MCF10A, reduced percentage of cells in S-phase, which was increased by Notch activation. IRF6 knockdown induced early apoptosis and reduced BCSCs, however it has no effect downstream of Notch in these processes. On the other hand, IRF6 did not play an essential role on expression of luminal and basal markers. In conclusion, our previous observation was supported that IRF6 is a mediator of Notch in cell proliferation. Furthermore, these data showed that IRF6 has a novel role on early apoptosis and stem cell population independent of Notch signaling.