Specifically, BCRP1/ABCG2, an ABC transporter, has been shown to play a major role in development of the SP phenotype in stem and progenitor cells during murine hematopoiesis and spermatogenesis and in human cancer stem-like cells (Zhou et al., 2001; Lassalle et al., 2004; Bleau et al., 2009). Here, ABCG2 is linked to cancer.