ABCG2 and cancer: For example, in melanoma, cancer stem cell markers include ABCB5, ALDH1, CD20, CD133 and CD271.19,20 For lung cancer, reported stem cell markers include ABCG2, ALDH1, CD90, CD117 and CD133.21 Breast stem cell markers include ALDH1, CD24, CD44, CD90, CD133 and α6–integrin.22 Reported colon cancer stem cell markers include ABCB5, ALDH1, β-catenin, CD24, CD26, CD29, CD44, CD133, CD166 and LGR5.23 However, these markers must be utilized with caution, i.e. they do not necessarily identify all cancer stem cells or any specific one.