This mutation was induced by X-ray irradiation of spores and selected for tolerance of gametophytes to the active ingredient in the herbicide Roundup®. This is a very popular, effective and widely used herbicide that interferes with aromatic amino acid synthesis. The active ingredient, glyphosate [N-(phosphonomethyl) glycine], competitively inhibits an enzyme (EPSP synthase) within this metabolic pathway. A number of glyphosate-tolerant mutants have been identified in several plant and microbial strains. Some of these mutants exhibit an altered form of EPSP synthase that is not competitively inhibited by the herbicide. In the case of this C-Fern mutation, the mechanism of tolerance is not known. Tolerance and sensitivity in the gametophyte generation can be observed when mutant and wild type gametophytes, respectively, are grown on nutrient agar medium containing RoundUp®. A 1:5 dilution of a standard (18%) commercial solution, filter sterilized, can be added to autoclaved medium at a rate of 30 – 100 microliters per 100 ml. If one-week-old gametophytes are transferred to this medium, differential tolerance of wild type and glt1 can be observed after 1-2 weeks of culture. Available only in a double mutant herbicide-tolerant line with pq45.