This mutation, which was induced by X-rays, confers tolerance to 2'-deoxy-5-fluorouridine (FUdR). This substance is a pyrimidine nucleoside analog and is highly toxic to many organisms. As a nucleotide analog, FUdR interferes with normal metabolism and nucleic acid biosynthesis. This mutation confers a very clear tolerance to FUdR and when gametophytes carrying it are grown in medium containing 10 mM FUdR, wild type gametophytes are killed shortly after germinating while mutant gametophytes grow quite normally.