Temperature sensitive - maintain at 15C. Recessive loss-of-function stronger with increases in temperature, nearly WT at 15C. At the non-permissive temperature DNA replication continues in the absence of mitosis. Mutants enter mitosis at the normal time and form bipolar spindles, but fail chromosome alignment at the metaphase plate, sister chromatid separation and cytokinesis. Molecular lesion is a 9 bp duplication followed by a T to C transversion. Mutagen was EMS but mutation likely caused by polymerase slippage.