Many cytologically normal and rearranged cut mutants have been reported, but no known deficiency involves both ct and its close neighbor, singed. This fact prompted an investigation of the mutational response of the ct-sn interval. Approximately 24,000 F(1) female progeny of 7-day-old males given 2000 or 3000r X-ray exposures were examined for the presence of newly induced mutations at the Notch, carmine, ct, and sn loci. One sn, 2 cm-ct, 31 N, and 33 ct mutants were found, indicating that the frequency of recovery of ct mutants is much greater than that of either cm or sn, as high even as N. Among the F(1) female progeny were two deficiency mutants that expressed both cm and ct (separated by 21 bands), but none expressed both ct and sn (separated by only 14 bands). Of the 18 cytologically analyzed ct mutants, two proved to be deficiencies; neither extended farther to the right than 7C1. No reported ct deficiency extends with certainty farther to the right than 7C4. This fact, together with the scarcity of sn deficiencies, suggests the presence of a haplo-insufficient locus between ct and sn that prevents recovery of ct-sn deficiencies. The analysis of the deficiency component of Tp sn(S93), a short transposition which moves most of the ct-sn interval from 7BC to 8D, proved the existence, just to the left of sn, of a haplo-inviable locus that prevents the development of females heterozygous for its deficiency.-A marked similarity between mutants at the N and ct loci was noted.