Following u.-v. treatment, a mutant strain of Neuro-spora #33757 has been isolated which is incapable of synthesizing leucine. Occasionally, cultures of leucineless Neurospora will "adapt". By genetic analysis of crosses between adapted and wild strains, it has been shown that adaptation depends on the mutation, or reversion, of the leucineless gene to an allele capable of mediating leucine synthesis. An analogy for cancer is suggested[long dash]the newly found capacity of a cell to synthesize an essential metabolite otherwise available only in limiting and regulatory amts.