Addition of heterochromatin suppresses while subtraction enhances position effect variegation. The heterochromatin-sensitive period has been determined in white/white-apricot variegated eyes of YSWa/Wa; Dp (1,3)W264-58 flies. When such larvae, carrying a Y-short (YS) arm at the distal end of one X chromosome, are X-rayed, mitotic recombination leads to one daughter cell with two YS arms and an adjacent daughter cell with no YS arm. When induced after clonal initiation, the frequency of dark clones developing from daughter cells with two YS arms is significantly higher than the frequency of dark clones in the rest of the eye; and this frequency is even higher when induced before clonal initiation. The modifying action of the Y-heterochromatin is exerted, therefore, during and after clonal initiation. Surprisingly, the frequency of dark clones developing from cells with no YS arm is not lower than the frequency of dark clones in the rest of the eye.