PRRT2 and cancer: While eCCA and iCCA share some common mutations (e.g., TP53, BRCA1, BRCA2, PK3CA, KRAS, SMAD4, ARID1A, GNAS), others are especially typical for small duct-iCCA (IDH1, IDH2, and BAP1 mutations as well as translocations involving FGFR2, NRG1, ALK, NTRK1-3, and possibly others; Table 3) and may eventually allow for the identification of iCCA in a cancer of unknown primary (CUP) constellation [16, 17].