The rap (retina aberrant in pattern) gene encodes the Fizzy-related protein (Fzr), which as an activator of the ubiquitin ligase complex; APC/C (anaphase promoting complex/cyclosome) facilitates the cell cycle stage-specific degradation of cyclins. Loss-of-function mutations in rap cause unscheduled accumulation of cyclin B in the developing eye imaginal disc, resulting in additional mitotic cycles and defective patterning of the developing Drosophila eye. Targeted mis-expression of rap/fzr in the eye primordial cells causes precocious cell cycle exit, and smaller primordial eye fields, which either eliminate or drastically reduce the size of the adult eye. Although mitosis is inhibited in the mis-expression animals, cells with abnormally large nuclei form tumor-like structures from continued endoreplication, cell growth and retinal differentiation. Interestingly, overexpression of Rap/Fzr in the eye primordia also increases the size of the antennal primordium resulting in the induction of ectopic antennae. These results suggest that Rap/Fzr plays an essential role in the timely exit of precursor cells from mitotic cycles and indicate that mechanisms that regulate cell cycle exit are critical during pattern formation and morphogenesis.