Cancer stem cells (CSC), which express CD133 [76], CD44, ATP-binding cassette sub-family G member 2 (ABCG2) and Aldehyde dehydrogenases (ALDH) [77], are a subpopulation of tumor cells that display self-renewal and the ability to give rise to heterogeneous lineages of cancer cells. Here, ABCG2 is linked to cancer.