Polymerase chain reaction (PCR) amplification of mtDNA from plasma and synovial fluid (SF) samples of patients with rheumatoid arthritis (RA). SDS–polyacrylamide-gel electrophoresis of PCR-amplified mitochondrial DNA (mtDNA) fragments (453 bp) of plasma and SF of five patients with RA. A semi-quantitative method was used to assign the PCR product intensities to positive or negative categories. An arbitrary cutoff point was assigned that was relative to the intensity of the 676-bp band (arrow) of the molecular mass marker on the gel. Lane 1, molecular weight marker; lane 2, water only (control); lane 3, plasma from patient 1 (plasma-1); lane 4, SF from patient 1 (SF-1); lane 5, plasma-2; lane 6, SF-2; lane 7, plasma-3; lane 8, SF-3; lane 9, plasma-4; lane 10, SF-4. The samples in lanes 11 (plasma-5) and 12 (SF-5) were spiked with 10 ng of purified mtDNA.