The severity of the headless phenotype produced by animals expressing sv^Scer\UAS.cJa under the control of Scer\GAL4^ey.PH in clones depends upon the time of the initial activation of expression of sv. The most severe phenotype, class I headless flies (lacking all head structures derived from the eye-antennal discs) are produced only if expression is induced before 12 hours after egg laying (AEL). Class II flies (eyeless flies which lack most head structures and both antennae) are seen only if expression is induced before 16 hours AEL. Class III flies (eyeless animals with large parts of the head missing but one or both antennae present) are only seen if expression is induced before the end of embryogenesis. Induction of sv expression after embryogenesis produces no headless flies. Most flies obtained after expression is induced 24-48 hours AEL are wild type, while a few have one or rarely more rough clones in the eye (suggesting that most clones expressing sv in the eye are lost). Induction of expression during the second or early third instar produces adults with mosaic eyes whose mutant clones are not rough.