An assay was designed that allows detection, by PCR alone, of the mutation of base pair no. 11,778 in human mitochondrial DNA, causing Leber's hereditary optic neuropathy. This was obtained by using a 20-mer primer with the mutation-specific base in the 3'-position, plus a deliberately introduced C/C-mismatch at base no. four from the 3'-end. The latter mismatch was necessary, and sufficient, to prevent amplification of the normal allele.