Pyogenic sterile arthritis, pyoderma gangrenosum, and acne syndrome: PAPA syndrome is an autosomal dominant autoinflammatory disorder caused by mutations in the PSTPIP1 gene. It results in recurrent, destructive, early-onset inflammation of the joints, skin and muscle that occurs without discernible infection. Biopsies of synovial tissue show massive polymorphonuclear, neutrophil-rich infiltrates without immunoglobulin or complement deposits. The episodic inflammatory arthritis typically does not resolve spontaneously and must be treated with intraarticular steroids or surgical drainage of the infiltrate. Some affected individuals have expanding inflammatory, ulcerative skin lesions (pyoderma gangrenosum) and/or severe acne conglobata consisting of many painful abscesses and draining sinuses that heal with scarring. Pathergy (injection site abscess) is also common.