THE PRIMARY SITE OF ACTING IS ENZYME RIBONUCLEOSIDE DIPHOSPHATE REDUCTASE. THE DRUG IS SPECIFIC FOR S PHASE OF THE CELL CYCLE AND CAUSES CELL TO ARREST AT THE G1-S INTERFACE. SINCE CELLS ARE HIGHLY SENSITIVE TO IRRADIATION IN THE G1PHASE OF THE CYCLE, COMBINATION OF HYDROXYUREA AND IRRADIATION CAUSE SYNERGISTIC TOXICITY.