To overcome the problem of initial period of sterility, the Russell group showed that instead of injecting one large dose of ENU, a fractionated dose (100 mg/kg) on a weekly schedule permitted a total higher dose (300–400 mg/kg) to be tolerated. This further showed that the mutation frequency improved to be 12 times that of X-rays, 36 times that of procarbazine and over 200 times that of spontaneous mutations. When the mutation rate was averaged across all 7 loci, ENU was found to induce mutations at a frequency of one per locus in every 700 gametes.