The ventral veinless gene (vvl) encodes the previously identified Cf1a protein, a transcription factor containing a POU-domain. During embryonic development vvl function is required for the formation of the tracheal tree and in the patterning of the ventral ectoderm. During imaginal development vvl is required for cell proliferation and the differentiation of the wing veins. vvl expression is restricted to the regions where its function is required, and is dependent on the coordinate activities of signalling molecules such as decapentaplegic, wingless and hedgehog. vvl interacts with other genes involved in vein differentiation, including veinlet, thick veins, torpedo, decapentaplegic and Notch suggesting that vvl function may affect several cell-to-cell communication pathways. We propose that the gene vvl integrates information from different signalling molecules and regulates the expression of specific cell differentiation genes during tracheal development and vein differentiation.