Heterozygotes are WT and segregate WT, Stu and DpyUncs. n3066 is a strong loss-of-function or null allele. Molecular lesion: ochre mutation terminating translation at amino acid 538. 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.