Based on the presence of multiple proline-rich motifs in the huntingtin sequence, we tested its possible association with epidermal growth factor (EGF) receptor signaling complexes through SH3 domain-containing modules. We found that huntingtin is associated with Grb2, RasGAP, and tyrosine-phosphorylated EGF receptor. These associations are regulated by activation of the EGF receptor, suggesting that they may be part of EGF receptor-mediated cellular signaling cascade. In vitro binding studies indicate that SH3 domains of Grb2 or RasGAP are required for their binding to huntingtin. Our results suggest that huntingtin may be a unique adapter protein for EGF receptor-mediated signaling and may be involved in the regulation of Ras-dependent signaling pathways.