We support the hypothesis that MTHFD1 overexpression related to a lower cellular redox status leads to gemcitabine chemoresistance in CCA cells, which is highly associated with the much lower cell viability of HyPer-high than HyPer-low CCA cells under gemcitabine treatment, indicating that MTHFD1 expression is highly correlated with gemcitabine chemoresistance in CCA cells (Fig. 6). Here, MTHFD1 is linked to cholangiocarcinoma.