Atypical bHLH transcription factor that regulates photomorphogenesis through modulation of phytochrome (e.g. PHYA) and cryptochrome signalings (Ref.4, PubMed:11090209, PubMed:10995393, PubMed:19482971). Suppresses the transcriptional regulation activity of PIF4 by forming non-DNA-binding heterodimer. Binds to FHY1 and FHL. Forms PHYA/FHY1/HFR1 complex (PubMed:19482971). Homodimer and heterodimer with PIF3. Do not interact alone with either phytochrome A (phyA) or B (phyB), but REP1/PIF3 complex binds to phyA and phyB, preferentially to the Pfr forms. Forms non-functional heterodimer with PRE6, causing liberation of PIF4 from the transcriptionally inactive complex HFR1-PIF4. Repressed when bound to PRE1, PRE2 and PRE4. Expressed constitutively in roots, leaves, stems, and flowers. Twofold induction by far-red light and 14-fold suppression by red light. Partially blind to far-red (FR). Impaired inhibition of hypocotyl elongation and cotyledons expansion under continuous FR light conditions. Contains a degenerate basic motif not likely to bind DNA.