Legs, Eyes, or Wings--Selectors and Signals Make the Difference

How do the cells that give rise to structures such as the eye, wing, and leg in the developing fruit fly know where they are and what to do? As Affolter and Mann explain in their Perspective, it now seems that a selector transcription factor complex and a signaling effector protein interact together on a DNA enhancer sequence, switching on the appropriate set of target genes that then direct development of the appropriate structure.