Strong allele. The size of photoreceptor cells in homozygous clones in the eye is reduced compared to heterozygous cells adjacent to the clone. Mutant clones in the wing imaginal disc and in the eye disc anterior to the morphogenetic furrow are similar in size to their wild-type sister clones. Mutant clones posterior to the morphogenetic furrow are significantly smaller than their wild-type sister clones, although significant differences in cell death and mitosis are not seen between mutant and control clones. Generation of flies in which the eye imaginal disc is homozygous for lilli4U5 (using the "ey-Flp" system) results in flies in which the head and eyes are reduced compared to wild type. The reduction in eye size is caused by a reduction both in the number and size of ommatidia (24% and 18% respectively).