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Scl 70 (topoisomerase 1) is a nuclear and nucleolar enzyme. The etymology of scl-70 consists of an abbreviation of scleroderma and a 70 kD extractable immunoreactive fragment that can be obtained from the otherwise larger (100-105 kD) target topoisomerase antigen (called the SCL-70 Antigen) of the antibodies. Anti-Scl-70 (also called anti-topoisomerase I after the type I topoisomerase target) is an anti-topoisomerase antibody-type of anti-nuclear autoantibodies, associated with more severe scleroderma disease. It was seen mainly in diffuse systemic scleroderma ,but is also seen in 10-18% of cases of the more limited form of systemic scleroderma called CREST syndrome. Scl 70 antibodies are more common in patients with extensive cutaneous involvement and interstitial pulmonary fibrosis, and are considered a poor prognostic sign.