Null alleles of the odz pair-rule gene have been generated as small Deficiency chromosome mutations. The true null phenotype of odz in segmentation is now seen to be very similar to that originally characterized, with each odd segment removed. No other previously isolated mutations in the genomic region proved allelic to odz. The generated Deficiencies covering the odz locus and immediately surrounding regions do not cover any other gene tested from that region. The entire odz gene has been cloned and mapped, and represents more than 120 kb of genomic DNA. The gene has been sequenced, except for two very large introns. Analysis of the upstream control region of the gene indicates the presence of a large number of putative binding sites for transcription factors that direct relevant developmentally regulated gene transcription.