The minisatellite DNA Pc-1 consists of tandem repeats of d(GGCAG). We previously reported that a d(GGCAG)n strand folds into an intramolecular quadruplex under physiological conditions and that during replication the progression of DNA polymerase is blocked by the quadruplex in vitro. Therefore, the formation of the quadruplex was supposed to be responsible for the hypermutable features of Pc-1. Then, we have identified proteins that bind to Pc-1, one of which is hnRNP A1. Here, we have demonstrated that hnRNP A1 destroys the quadruplex of Pc-1 on binding and abrogates the arrest of DNA polymerase at the repeat. Thus, hnRNP A1 functions as if it is a chaperon to assist Pc-1 DNA to form the proper folding suitable for replication. We have also found that hnRNP A1 and a related protein, hnRNP D, destroy the quadruplex of telomere DNA, which suggests the involvement of these proteins in telomere maintenance as DNA chaperons.