Previously, Wilson et al. (2006) reported that Clusters 7 and 8 of Valk et al. (2004) had a gene expression profile most similar to cluster B. It has been noted that one of genes overexpressed by these patients was ABCG2, a member of ATP-binding cassette (ABC) superfamily of membrane transporters, also termed breast cancer resistance protein (BCRP1), later dubbed as a stem cell marker and a target in cancer stem cell therapy (Ding et al., 2010). The gene discussed is ABCG2; the disease is cancer.