Fission yeast cut5 mutants cause cytokinesis in the absence of normal nuclear division. We show here that cut5+ is required for both the onset of S phase and the restraint of M phase before the completion of S phase. The primary defects in cut5 mutants occur prior to S phase, but cells suffer lethal damage during M phase. Mitosis and cytokinesis occur in the presence of hydroxyurea or in the double mutant cdc10-cut5 (the cdc10 mutation alone blocks progression from G1 to S). Gene cloning shows that cut5+ is identical to the fission yeast rad4+ gene, which is similar to human XRCC1. The rad4+/cut5+ gene is unique in its positive role for replication/repair and in its negative role for mitosis/cytokinesis. We propose a single/twin chromatid marking model for rad4+/cut5+ function in cell cycle control.