Essential machinery. We found that normal yeast cells lose a chromosome or undergo mitotic recombination at rates of about one event in 105 cells. We wondered whether limitation of any of the essential cell cycle functions would alter that fidelity. An undergraduate, David Smith and I studied chromosome loss, recombination and mutation in temperature-sensitive cell cycle mutants when they were grown at their maximum permissive temperature. We found that most mutants had greatly elevated rates of chromosome loss, recombination or mutation, implying that defects in these functions could be important in the fidelity of cell division.