However, the definitive identification of clinically relevant cancer subgroups that ultimately encumber patient survival through drug-resistance and metastatic spread remains a challenge, in part because commonly used stem cell markers (e.g. CD133, CD44, and CD166) may not accurately reflect these biological characteristics.26 Some markers may not be expressed at the protein level, for example, LGR5.27 The gene discussed is PROM1; the disease is cancer.