Fig. 4. Su(z)12 mutations suppress position-effect variegation. Heads of adult flies that are hemizygous for the wm4 rearrangement. In wild-type flies (left), pigmentation in eyes is drastically reduced owing to silencing of the white (w) gene in most ommatidia. In animals that are heterozygous for any of the Su(z)12 alleles (right), there is a partial release from silencing and the w gene is expressed in most ommatidia; the loss of silencing is stronger in case of the EMS-induced Su(z)12 alleles. In each case, the wild-type (+/+) control flies on the left carry the TM3 balancer chromosome and are the siblings of the corresponding Su(z)12 mutants.