Birt-Hogg-Dube syndrome (BHD; MIM 135150) also known as Hornstein-Knickenberg syndrome, is an autosomal dominant genodermatosis. It is characterized by cutaneous findings (fibrofolliculomas and acrochordons), pulmonary cysts and spontaneous pneumothorax, and renal cysts and tumors. Additional findings may include facial papules, intestinal polyposis, colorectal adenomas,  parotid oncocytomas, neural tissue tumors, lipomas, and angiolipomas. BDH is caused by mutations in the FLCN gene, which codes for folliculin. Some individuals with primary spontaneous pneumothorax (PSP; MIM 173600) can have dominant mutations in FLCN. Most of these individuals also have bullous lung lesions.  Spontaneous pneumothorax can be a finding in certain connective tissue disorders including Marfan syndrome, type I (MFS1; MIM 154700) and Ehlers-Danlos syndrome, type IV (EDS IV; MIM 130050). Pulmonary bullae can also occur with alpha-1-antitrypsin deficiency (MIM 613490).