ABCG2 and prostate cancer: To address the possibility that stable inhibition of GSK-3 leads to selection for prostate cancer stem/progenitor-like cells, as was recently reported for PC3 cells [28], we measured the expression levels of three genes expressed in 22Rv1-derived stem/progenitor cells, SOX2, NANOG and BCRP [29], in the 22Rv1-derived cell clones.