The basic/helix-loop-helix (bHLH) proteins are a superfamily of transcription factors that bind as dimers to specific DNA sites and are well characterized in non-plant eukaryotes. This study identifies 147 bHLH protein-encoding genes in Arabidopsis, making it one of the largest transcription factor families. Phylogenetic analysis classifies these genes into 21 subfamilies. The bHLH domain consists of a basic region (involved in DNA binding) and an HLH region (dimerization domain). These proteins exhibit sequence divergence outside the conserved bHLH domain and can form homodimers or heterodimers. The core DNA binding motif is the E-box (5'-CANNTG-3'), with the G-box (5'-CACGTG-3') being the most common. The study shows that PIF3 and PIF4, two related phytochrome-interacting bHLH proteins, can form homodimers and heterodimers that bind specifically to the G-box. These findings support the idea that bHLH proteins can participate in combinatorial interactions, regulating multiple transcriptional programs. The AtbHLH proteins are distributed across the Arabidopsis genome, with some regions showing higher gene density. The family is divided into 21 subfamilies based on phylogenetic analysis, with some subfamilies showing conserved intron distribution patterns. The bHLH domain has varying DNA binding properties, with some proteins predicted to bind DNA (E-box binders) and others not (non-DNA binders). The study also shows that some AtbHLH proteins may have a recent common evolutionary origin, possibly due to genome duplication events. The results suggest that the Arabidopsis bHLH family has diverse functional activities, with some proteins acting as negative regulators of E-box binding bHLHs through heterodimerization. The study provides evidence that AtbHLH proteins can form heterodimers capable of sequence-specific DNA binding, similar to their animal counterparts. The analysis of AtbHLH proteins in comparison to other eukaryotes shows that Arabidopsis has a larger number of bHLH proteins than many other organisms, with the most abundant type being putative G-box binders. The study highlights the importance of bHLH proteins in plant biology, particularly in phytochrome-regulated light signaling pathways.The basic/helix-loop-helix (bHLH) proteins are a superfamily of transcription factors that bind as dimers to specific DNA sites and are well characterized in non-plant eukaryotes. This study identifies 147 bHLH protein-encoding genes in Arabidopsis, making it one of the largest transcription factor families. Phylogenetic analysis classifies these genes into 21 subfamilies. The bHLH domain consists of a basic region (involved in DNA binding) and an HLH region (dimerization domain). These proteins exhibit sequence divergence outside the conserved bHLH domain and can form homodimers or heterodimers. The core DNA binding motif is the E-box (5'-CANNTG-3'), with the G-box (5'-CACGTG-3') being the most common. The study shows that PIF3 and PIF4, two related phytochrome-interacting bHLH proteins, can form homodimers and heterodimers that bind specifically to the G-box. These findings support the idea that bHLH proteins can participate in combinatorial interactions, regulating multiple transcriptional programs. The AtbHLH proteins are distributed across the Arabidopsis genome, with some regions showing higher gene density. The family is divided into 21 subfamilies based on phylogenetic analysis, with some subfamilies showing conserved intron distribution patterns. The bHLH domain has varying DNA binding properties, with some proteins predicted to bind DNA (E-box binders) and others not (non-DNA binders). The study also shows that some AtbHLH proteins may have a recent common evolutionary origin, possibly due to genome duplication events. The results suggest that the Arabidopsis bHLH family has diverse functional activities, with some proteins acting as negative regulators of E-box binding bHLHs through heterodimerization. The study provides evidence that AtbHLH proteins can form heterodimers capable of sequence-specific DNA binding, similar to their animal counterparts. The analysis of AtbHLH proteins in comparison to other eukaryotes shows that Arabidopsis has a larger number of bHLH proteins than many other organisms, with the most abundant type being putative G-box binders. The study highlights the importance of bHLH proteins in plant biology, particularly in phytochrome-regulated light signaling pathways.