AKR1C4 and lung adenocarcinoma: The expression of AKR1C1, AKR1C2, AKR1C3, and AKR1C4 proteins, which was mainly overexpressed in lung adenocarcinoma (A549) cells [35] and associated with drug resistance in NSCLC [36,37], was found in the A549 CD166 + EpCAM + CSC subpopulation.