PROM1 and head and neck squamous cell carcinoma: For example, PD-L1 expression is increased in CD44+ breast cancer stem cells (Wu et al., 2017; Hsu et al., 2018), CD133+ colorectal cancer stem cells (Zhi et al., 2015; Wu et al., 2017), and CD44+ head and neck squamous cell carcinoma (HNSCC) stem cells; VTCN1 expression is also elevated in CD133+ glioblastoma stem cells.