About one-third of the more than 300 C2H2 zinc finger proteins of Drosophila contain a conserved sequence motif, the zinc finger-associated domain (ZAD). Genes that encode ZAD proteins are specific for and expanded in the genomes of insects. Only three ZAD-encoding gene functions are established, and the role of ZAD is unknown. Here we present the crystal structure of the ZAD of Grauzone (ZAD(Grau)), a Drosophila transcription factor that specifically controls the maternal Cdc20-like APC subunit Cortex. ZAD forms an atypical treble-clef-like zinc-coordinating fold. Head-to-tail arrangement of two ZAD(Grau) molecules in the crystals suggests dimer formation, an observation supported by crosslinking and dynamic light scattering. The results indicate that ZAD provides a novel protein-protein interaction module that characterizes a large family of insect transcription factors.