AbstractAdditional sex combs (Asx) is thought to function in protein complexes of both the Trithorax and Polycomb groups, but very little is known about its developmental roles. Here, we present a detailed analysis of Asx's role in antennal development. We show that loss of Asx in the antennal disc causes a complex phenotype, which consists of distal antenna‐to‐leg transformations and outgrowth of ectopic leg‐like appendages from the Dpp‐expressing domain of the disc. Our analyses suggest that these phenotypes are caused mainly by segment‐specific de‐repression of Antp and expansion of wg expression. We thus conclude that Asx functions normally to repress Antp and to restrict wg expression in specific regions of the developing disc. We also show that, in the absence of Asx's function, Antp expression does not lead to efficient repression of the antennal‐determining gene hth, suggesting that Asx is also required for the repression of hth by Antp. Developmental Dynamics 236:2118–2130, 2007. © 2007 Wiley‐Liss, Inc.