We have investigated (i) the role of pdm1, a Drosophila POU gene, during the elaboration of the GMC-1-->RP2/sib lineage and (ii) the functional relationship between pdm1 and the closely linked second POU gene, miti-mere, in this lineage. We show that deletion of pdm1 causes a partially penetrant GMC-1 defect, while deletion of both miti and pdm1 results in a fully penetrant defect. This GMC-1 defect in miti- and pdm1- embryos can be rescued by the pdm1 or miti transgene. Rescue is observed only when these genes are expressed at the time of GMC-1 formation. Overexpression of pdm1 or miti well after GMC-1 is formed results in the duplication of RP2 and/or sib cells. Our results indicate that both genes are required for the normal development of this lineage and that the two collaborate during the specification of GMC-1 identity.