A mutant, YS17, at the buff spore colour locus in Sordaria brevicollis has previously been described. It shows conversion with high frequency, predominantly to wild type, and is believed to act as a recognition site for an endonuclease that initiates recombination at the YS17 site. The discovery is now reported of a gene that causes loss of the high-frequency conversion shown by the YS17 mutant. The gene was present in existing stocks of the fungus. It reduces the conversion frequency of YS17 to a level similar to that of other buff mutants, from which it is inferred that the YS17 mutant no longer acts as an initiation site for recombination. When the conversion frequency of YS17 is low the bias in conversion to wild type rather than to mutant is lost, suggesting that this bias may relate to the initiation of recombination at the site. The loss of high frequency conversion of YS17 appears to be determined by a single recessive gene linked to mating type and unlinked to buff. It is suggested that the dominant allele induces recombination at the site of YS17 by controlling either the synthesis or the activity of an endonuclease that is capable of recognising the nucleotide sequence at the YS17 site. Some anomalous results point to the existence of modifiers of the action of the gene.