Transcription factor that binds to a symmetrical DNA sequence (E-boxes) (5'-CACGTG-3') that is found in a variety of viral and cellular promoters. Interacts with MAF (By similarity). Efficient DNA binding requires dimerization with another bHLH protein. Binds DNA as a homodimer or a heterodimer (USF1/USF2). In vivo, the USF1/USF2A heterodimer represents over 66% of the usf binding activity whereas the USF1 and USF2A homodimers represent less than 10%. The USF1/USF2B heterodimer accounted for almost 15% in some cell. Additional isoforms seem to exist. Ubiquitous. Can bind as a homodimer to the E-box of the cathepsin B (CTSB) promoter.