PROM1 and cancer: Genomic studies of GBM heterogeneity have revealed the presence of a CD133+ chemo- and radio-resistant cancer stem cell (CSC) population responsible for tumor initiation and found to have higher expression in recurrent tumors (Bao et al., 2006; Liu et al., 2006; Tamura et al., 2013; Qazi et al., 2017), but even then, CD133+ has failed to be a universal marker of CSCs given the discovery of similarly functioning CD133- cell populations (Chen et al., 2010).