Zygotic homozygosity for Borr^E133 results in late embryonic lethality, although the mutant embryos lack overt morphological defects. By stage 12, cells in the ventral nerve cord and brain exhibit abnormally large nuclei, on average approximately 3 times larger than those of wild-type. Borr^E133 mutant clones induced during early larval stages are rare in fully grown larval discs, and are much smaller than the corresponding wild-type twin spots, suggesting that a large fraction of the mutant cells die. However, many of the surviving Borr^E133 mutant cells are large, with giant but well-formed nuclei that appear healthy, and well integrated into the epithelial tissue. Borr^E133 clone cells exhibit abnormal mitotic spindles, including multipolar ones. Borr^E133 mutant clone cells appear to undergo apoptosis and are cleared from the tissue by basal extrusion from the epithelium. The frequency of surviving Borr^E133 mutant clone cells is increased if they are induced in a Minute background, which provides the cells with a proliferative advantage. They can thus occupy a significant fraction of imaginal disc territories in third instar larvae. All discs are equally affected, and they tend to be smaller than wild-type discs of an equivalent stage. Larvae with these clones do not survive pupariation. Adults derived from first or second instar larvae in which Borr^E133 mutant clones have been induced exhibit abnormal legs and rough eyes. In addition they exhibit large wing nicks. In all these cases, a twin spot is apparent, but no mutant tissue is detectable, indicating that by the adult stage, each of the early-induced Borr^E133 mutant cells has undergone apoptosis. Flies in which Borr^E133 mutant clones are induced from early third instar larval stage onwards are viable and display no gross patterning defects. Late-induced Borr^E133 mutant clones occupy the same amount of territory as their wild-type twin-spots, indicating that the mutant cells have survived three or four consecutive (abnormal) mitoses without entering the apoptotic pathway. The wing blades of Borr^E133 mutant clones contain clusters of hairs (trichomes) surrounded by large clearings, rather than the usual regularly spaced single hairs. The number of hairs per cluster varies, with the largest clusters consisting of 12 hairs. All these hair clusters are produced by Borr^E133 mutant cells, indicating a cell-autonomous phenotype. Borr^E133 mutant clones do not significantly affect the planar polarity in the wing blade as mutant and surrounding wild-type hairs appear normally oriented. Borr^E133 mutant clones induced in pupal wing discs occupy roughly the same amount of territory as their twin spots. In addition, no nuclei were observed with compacted DNA, indicating imminent apoptosis. The surviving Borr^E133 mutant cells in the pupal discs are much larger than their neighbours, often with giant nuclei, indicating a high degree of ploidy. Borr^E133 mutant cells appear healthy and are well integrated within the epithelial tissue. Late-induced Borr^E133 mutant clones exhibit abnormal giant bristles, invariably lacking sockets, in the wing margins.