ABCG2 and prostate cancer: A series of seminal papers published in 2005 provided the earliest evidence for the existence of CSCs in prostate cancer: first, a population of tumorigenic cells with high expression of the ABCG2 drug efflux pump (termed the side population) was isolated in human prostate cancer LAPC9 xenografts [34]; second, ABCG2 was discovered to mediate the efflux of androgen in putative CSCs [35]; and third, cells isolated from prostate tumors based on the cell surface markers CD44+/CD133+/α2β1+ could self-renew and differentiate to generate a phenotypically mixed population [29].