Fig 3. Testing for CPE in flies.

Eyes of 2 to 3 day old heterozygous females were photographed before (M2) and after (ΔM2) flipping out the M2 insulator upstream of the mini-w gene to test for CPE. Heterozygotes were used to avoid any confounding pairing effects, although CPE is equally apparent in homozygotes. The fly lines shown were used to make attP sites. Note that the 3W.3 line shows reverse CPE, suggesting the M2 insulator is blocking a silencer rather than an enhancer.