FLCN and Pulmonary bulla: Positive expression of FLCN in epithelial and mesenchymal cells was seen in both Birt–Hogg–Dubé and simple pulmonary bulla patients, and the grayscale value of positive cells of FLCN in Birt–Hogg–Dubé patients was lower than in patients with simple pulmonary bulla (95.000 ± 1.546 vs. 143.067 ± 1.115, Figures 8(a) and 8(b)).