MTHFD1 and cholangiocarcinoma: The evidence of relatively higher MTHFD1 expression in the HyPer-low subpopulation CCA cells than in the HyPer-high cells combined with the higher gemcitabine chemoresistance in the HyPer-low CCA cells encouraged us to determine whether MTHFD1 serves as one of the key regulators linking the cellular redox status with gemcitabine chemoresistance.