An autoantibody specific for a DNA quadruplex structure has been isolated and cloned from three-month-old autoimmune "viable motheaten" mice. This antibody (mev-alpha Q1) has been tested extensively in vitro and found to bind specifically and preferentially to the parallel-stranded quadruplex structure formed by the oligonucleotide d(CGC G4 GCG). The anti-quadruplex antibody does not show specific affinity for single-, double-, or triple-stranded oligonuclotides of similar CG-rich sequence motifs.