ABSTRACT The Drosophila POU gene miti-mere (previously known as pdm2) has a complex spatial and temporal pattern of expression during early development; initially it is expressed in gap-gene-like pattern, then in 14 stripes and finally in a subset of the cells in the developing CNS and PNS. To study the function of this gene during develop-ment, we generated a ‘synthetic anti-morphic mutation’ by expressing a truncated version of the miti protein from a constitutive hsp83 and an inducible hsp70 promoter. We show that these Δmiti transgenes behave like classical anti-morphic mutations. Using these dominant negative trans-genes, together with deletions and a duplication for the gene, we show that miti is required during segmentation and neurogenesis. We have also used temperature-shift experiments with the hsp70Δmiti transgene to demonstrate that miti function in segmentation is distinct and separable from its function during neurogenesis. In segmentation, miti appears to be required in the specification of the segments A2 and A6. In the CNS, miti is required for the elaboration of the NB4-2→GMC-1→RP2/sib lineage. miti is initially required in this lineage to establish the identity of the parental ganglion mother cell, GMC-1. miti must then be down-regulated to allow the asymmetric division of GMC-1 into the RP2 and its sibling cell.