Mutations in components of the Hedgehog (Hh) signal transduction pathway underlie a number of human developmental disorders, and contribute to a diverse array of tumors. Our long-term goals are to elucidate how this signal transduction pathway is usurped in these different human pathologies. To achieve this goal we first have to understand how this pathway functions in a physiological setting, using Drosophila melanogaster as a model system for the more complicated mammalian Hh pathway. All Hh signaling, in both developmental and pathological settings, goes through the seven-transmembrane protein Smoothened (Smo). Consistent with Smo's pivotal role in the Hh signal transduction pathway, a number of small-molecule Smo antagonists are currently undergoing clinical trials for the treatment of Hh-dependent tumors. Thus, understanding the signaling events immediately downstream of Smo will have an immediate relevance to human health. We have identified the only known direct Smo effectors, the kinesin-like protein Costal2 (Cos2) and the heterotrimeric guanine nucleotide binding protein (G-protein) Gi, which we speculate regulate overlapping branches of the Hh signaling pathway that combine to regulate the activity and levels of the transcription factor Cubitus interruptus (Ci). Thus the goal of the proposed research is to identify the molecular mechanisms by which Cos2 and Gi signaling combine to regulate Hh signal transduction. Such information is essential to establish how Ci is regulated by Hh, and to understanding how this mechanism is applicable to the regulation of Ci homologs during human development and in various human pathologies associated with dysfunctional Hh signaling.