MTHFD1 and cholangiocarcinoma: To uncover the correlation between the intrinsic cellular redox status and CCA gemcitabine chemoresistance, we applied a novel tool, the HyPer3 probe, to sort out cellular redox status differences in a subpopulation of CCA cells and found that HyPer-low CCA cells exhibited higher gemcitabine chemoresistance than others with higher MTHFD1 expression.