P2 and P4 sera were obtained from patients with scleroderma withknown anti-topoisomerase I antibodies.42−46 MS analysis of the P2 serum immunoprecipitate showedthe presence of both cytoplasmic topoisomerase I (TOP1) and mitochondrialtopoisomerase 1 (TOP1MT), two highly homologous proteins (Figure 3B). Here, TOP1MT is linked to scleroderma.