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We show that mutation in polo leads to a variety of abnormal mitoses in Drosophila larval neuroblasts. These include otherwise normal looking mitotic spindles upon which chromosomes appear overcondensed; normal bipolar spindles with polyploid complements of chromosomes; bipolar spindles in which one pole can be unusually broad; and monopolar spindles. We have cloned the polo gene from a mutant allele carrying a P-element transposon and sequenced cDNAs corresponding to transcripts of the wild-type locus. The sequence shows that polo encodes a 577-amino-acid protein with an amino-terminal domain homologous to a serine-threonine protein kinase. polo transcripts are abundant in tissues and developmental stages in which there is extensive mitotic activity. The transcripts show no obvious spatial pattern of distribution in relation to the mitotic domains of cellularized embryos but are specifically …