Fig. 1.

Immunohistochemical analysis of CMPTs. a Low magnification view of a CMPT showing papillary epithelial proliferation with abundant mucus production. b Higher magnification view of the same case showing a mixture of goblet, ciliated columnar, and basal cells (arrow). c HNF-4α positivity in epithelial cell nuclei. Scale bar = 60 μm. d The positive rate for Ki-67 expression was <10%. Scale bar = 100 μm. e Focal MUC5AC staining in occasional ciliated cells. Scale bar = 90 μm. f Epithelial cells and mucin were strongly positive for MUC5B. Scale bar = 2 mm. g BRAF V600E staining was strong and diffuse in most epithelial cells of a case harboring BRAF V600E and AKT1 E17K mutations. Scale bar = 60 μm. h PD-L1 staining was mostly negative (< 1%); however, focal membranous immunoreactivity was observed. Scale bar = 70 μm