Human Anti-Scl-70 Antigen polyclonal antibody for ELISA. The Scl-70 antigen is an abundant nuclear protein susceptible to proteolysis. It has been shown that the Scl-70 antigen can be obtained as an extractable immunoreactive fragment (70 kD) of a larger protein (100-105kD), which has been shown to be Topoisomerase I. Anti-Scl-70 antibodies were originally described as precipitating autoantibodies in sera of patients with a connective tissue disease.