Transcription factor acting negatively in the phytochrome B signaling pathway. May regulate the expression of a subset of genes involved in cell expansion by binding to the G-box motif (By similarity). Activated by CRY1 and CRY2 in response to low blue light (LBL) by direct binding at chromatin on E-box variant 5'-CA[CT]GTG-3' to stimulate specific gene expression to adapt global physiology (e.g. hypocotyl elongation in low blue light) (PubMed:26724867). Interacts preferentially with the Pfr form of phytochrome B (phyB). Binds DNA as a homodimer, but once bound to DNA, loses its capacity to interact with phyB. Interacts with APRR1/TOC1 and PIF3. Binds to RGL2 and RGA. Forms non-functional heterodimer with HFR1. Interacts with PHYB, CRY1 and CRY2 in the nucleus in response to low blue light (LBL) (PubMed:26724867, PubMed:12826627, PubMed:12897250, PubMed:20093430, PubMed:23224238). Interacts with FYPP1 and FYPP3 (PubMed:31527236). By UV treatment. Follow a free-running robust circadian rhythm, with higher levels during the light phase. Rapidly induced by light in etiolated plants. Sixfold induction by both red and far-red light. Belongs to the bHLH protein family.