Heterozygotes are WT and segregate WT, Stu and DpyUncs. n3070 is a strong loss-of-function or null allele. Molecular lesion: P to S at position 24 as well as an amber mutation terminating translation after amino acid 52. DNA replication continues in the absence of mitosis. Mutants enter mitotis at the normal time and form bipolar spindles, but fail chromosome alignment at the metaphase plate, sister chromatid separation and cytokinesis.