YAP1 and breast cancer: To determine the cellular functions of YAP1 in promoting tumor metastasis and invadopodia formation, we first examined its endogenous expression and Ser127 phosphorylation levels (pS127-YAP1), a marker of YAP1 nuclear-cytoplasmic translocation and inactivation [26], in four breast cancer cell lines (MCF7, T47D, MDA-MB-231, and MDA-MB-468) and a non-tumorigenic breast epithelial cell line (MCF-10A) with relatively low and high cell density.