¹Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, MA 02115, USA

²Mental Health Research Institute and Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan, MI 48104-1687, USA

Correspondence to: D L Turner, Neuroscience Laboratory Building, 1103 East Huron, Ann Arbor, Michigan, MI 48104-1687, USA; E-mail: dlturner@umich.edu

The basic-helix-loop-helix (bHLH) proteins are a superfamily of DNA-binding transcription factors that regulate numerous biological processes in both invertebrates and vertebrates. One family of bHLH transcriptional repressors is related to the Drosophila hairy and Enhancer-of-split proteins. These repressors contain a tandem arrangement of the bHLH domain and an adjacent sequence known as the Orange domain, so we refer to these proteins as bHLH-Orange or bHLH-O proteins. Phylogenetic analysis reveals the existence of four bHLH-O subfamilies, with distinct, evolutionarily conserved features. A principal function of bHLH-O proteins is to bind to specific DNA sequences and recruit transcriptional corepressors to inhibit target gene expression. However, it is likely that bHLH-O proteins repress transcription by additional mechanisms as well. Many vertebrate bHLH-O proteins are effectors of the Notch signaling pathway, and bHLH-O proteins are involved in regulating neurogenesis, vasculogenesis, mesoderm segmentation, myogenesis, and T lymphocyte development. In this review, we discuss mechanisms of action and biological roles for the vertebrate bHLH-O proteins, as well as some of the unresolved questions about the functions and regulation of these proteins during development and in human disease. Oncogene (2001) 20, 8342-8357

basic-helix-loop-helix; transcription; Notch; corepressor; hairy