Sca‐1 is a mouse glycosylphosphatidylinositol‐anchored surface protein (GPI‐AP), initially used as a marker of hematopoietic stem cells (SC) (Bradfute et al., 2005; Holmes and Stanford, 2007) and more recently some populations of mammary tumor initiating cells (TIC) (Weng et al., 2012). Here, CASP3 is linked to breast cancer.