Inhibition of topoisomerase-I by anti-topo-I positive immunoglobulins from SSc sera. Topoisomerase-I was preincubated with ammonium sulphate precipitated immunoglobulins from patients’ sera prior to the addition of pUC19 plasmid DNA. Lane 1: DNA marker (1kb DNA Extension Ladder, Thermo Fisher Scientific, Waltham, MA, USA); lane 2 and 3 show controls: supercoiled pUC19 DNA migrating at approx. 1.600 bp (lane 2) and topoisomerase-I-induced relaxation of pUc19 DNA to approx. 3.500 bp (lane 3); lane 4 and 5 show the effect of incubation of an anti-topo-I-positive and an anti-topo-I-negative SSc serum with topoisomerase I on supercoiled pUC19 DNA: inhibition of relaxation in the presence of immunoglobulins from a SSc serum being anti-topo-I-positive by ELISA (lane 4) and no inhibition of relaxation in the presence of immunoglobulins from a SSc serum being anti-topo-I-negative by ELISA (lane 5)