For instance, in one of the articles identified by this systematic review, the gene expression levels of NANOG, a key regulator of pluripotency, and therefore a marker of stem cell-like behavior, was quantitatively tested along with other proteins (SOX2, OCT4, KLF4, ABCG2, CMYC, MSI1, CD44, NOTCH1, NES, SALL4B, TP53, and EPAS1) using Real Time-quantitative PCR (RT-qPCR) in 33 surgical specimens of low- (WHO grade I) as well as in high-grade (WHO grade II/III) meningiomas [12]. The gene discussed is ABCG2; the disease is meningioma.