Homozygous larval brains show an increased mitotic index (1.63) compared to wild type (1.04). 83% of the mitotic cells are in prometaphase/metaphase compared to 88% in wild type. 76% of anaphases show abnormalities, 15% of mitotic cells have overcondensed chromosomes and 78% of prometaphase/metaphase cells show precocious sister chromatid separation (PSCS). Metaphase figures show high levels of aneuploidy. Many chromosomes are still able to congress to the metaphase plate, but this congression is not normal; small chromatin entities are situated away from the metaphase plate in a large fraction of the cells and the chromosomes at the metaphase plate are less tightly arrayed than in wild type. Sister centromeres are correctly oriented towards the poles but are further apart than normal. Hemizygous larval brains show a reduced mitotic index (0.45) compared to wild type (1.04). 94% of the mitotic cells are in prometaphase/metaphase compared to 88% in wild type. 100% of anaphases show abnormalities, 86% of mitotic cells have overcondensed chromosomes and 98% of prometaphase/metaphase cells show precocious sister chromatid separation (PSCS). san¹/san^KG04816 larval brains show a reduced mitotic index (0.48) compared to wild type (1.04). 91% of the mitotic cells are in prometaphase/metaphase compared to 88% in wild type. 100% of anaphases show abnormalities, 68% of mitotic cells have overcondensed chromosomes and 82% of prometaphase/metaphase cells show precocious sister chromatid separation (PSCS).