PCR preferential amplification consists of the inefficient amplification of one allele in a heterozygous sample. Here, we report the isolation of a GC-rich human minisatellite, MsH43, that undergoes allelic preferential amplification during PCR. This effect requires the existence of a (TGGGGC)(4) motif that is able to form a G-quadruplex in the presence of K(+). This structure interferes with the DNA synthesis of the alleles harbouring this motif during PCR The present results are the first demonstration that the formation of G-quadruplex can be one of the mechanisms involved in some kinds of preferential amplification.