Scl-70 antigen is an abundant nuclear protein that is an extractable fragment (70 kD) of a larger protein (100-105kD), Topoisomerase I. Anti-Scl-70 antibodies are a serologic marker of diffuse cutaneous systemic sclerosis (dcSSc), a connective tissue disease characterized by microvascular damage and fibrosis of the skin and visceral organs. Autoantibodies to Scl-70 are also a specific serologic marker for systemic sclerosis and acrosclerosis, but are not usually present in other autoimmune rheumatic diseases unless some cutaneous symptoms of scleroderma are present.