Birt-Hogg-Dubé syndrome (BHDS, MIM 135,150), or Hornstein-Knickenberg syndrome, is an autosomal dominant hereditary disorder associated with a germline pathogenic variant folliculin (FLCN), which increases risk of benign cutaneous fibrofolliculomas, pulmonary cysts, spontaneous pneumothorax, and multiple, mainly malignant, bilateral and multifocal renal neoplasias. The gene discussed is FLCN; the disease is pneumothorax.