A single recessive nuclear gene mutation has been isolated from strain 123.1C of Saccharomyces cerevisiae which appears to be conditionally deficient in nuclear DNA metabolism. Growth of the mutant strain at the elevated temperature of 36 degree C results in rapid loss of cell viability. However, no apparent reduction in the rate of radioisotope incorporation into DNA was detected during this period. When haploid cells carrying this temperature sensitive lesion were exposed to the restrictive temperature for varying lengths of time, returned to the permissive temperature, mated with a non-temperature sensitive strain and then the resulting diploids made to undergo meiosis, a greatly reduced number of viable spores were produced. Genetic analysis of the viable spores produced by these diploids has revealed aberrant auxotrophic marker segregation patterns. Thus, these results suggest that the mutated gene hardbored in this strain plays a vital role in the metabolism of the nuclear genome.