Defects in the folliculin (FLCN) gene are the cause of Birt-Hogg-Dube syndrome (BHD) [MIM:135150]. BHD is a rare autosomal dominant genodermatosis characterized by hair follicle hamartomas (fibrofolliculomas), kidney tumors, and spontaneous pneumothorax. Fibrofolliculomas are part of the triad of BHD skin lesions that also includes trichodiscomas and acrochordons. Defects in FLCN may also be the cause of primary spontaneous pneumothorax (PSP) [MIM:173600], a condition in which air is present in the pleural space in the absence of a precipitating event, such as trauma or lung disease. Folliculin may be a tumor suppressor involved in colorectal and renal cell tumorigenesis [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/Q8NFG4].