MIR21 and cholangiocarcinoma: Previously, miRNAs have been shown to modulate drug sensitivity and to act as biomarkers of drug response.1, 4, 30, 31, 32, 33, 34, 35, 36, 37 MIR21 is an oncogenic miRNA that drives CCA pathogenesis and sensitization to conventional chemotherapy drugs.38, 39 Thus, we investigated if MIR21 could be used as a biomarker of response to HSP90 inhibition in CCA.