SOX9 and cholangiocarcinoma: We performed serial section immunostaining to detect HC and BEC markers, including HA-tag (AKT), YAP1, SOX9, HNF4α, panCK, and CK19, at 2 weeks and 5 weeks post-HDTVI, which correspond to the stages before the clonal expansion of transduced cells and after their transformation into mature CCA, respectively (Figure 4B,C) [13].