Auxin response factors (ARFs) are transcription factors that bind to auxin response elements in promoters of early auxin response genes and play central roles in many auxin-mediated processes. As transcription factors, ARFs possess a modular structure and consist of three major domains: an amino-terminal DNA-binding domain (DBD); a middle domain, which is responsible for gene activation or repression; and a carboxy-terminal PB1 (Phox and Bem1) domain, contained within a region that was previously called domain III/I. ARFs function as transcription activators or repressors, depending on the middle region of the ARF protein; Q-rich middle regions activate transcription, whereas P/S/T-rich middle regions confer repressor activity.