Recent studies have identified a family of DNA-binding proteins that share a common DNA-binding and dimerization domain with the potential to form a helix-loop-helix (HLH) structure. Various HLH proteins can form heterodimers that bind to a common DNA sequence, termed the E2-box. We demonstrate here that E2-box-binding B-cell- and myocyte-specific nuclear factors contain subunits which are identical or closely related to ubiquitously expressed (E12/E47) HLH proteins. These biochemical data suggest a pleiotropic function for E12/E47-like molecules in mammalian differentiation, similar to the genetically defined function of daughterless in Drosophia development. One class of DNA-binding proteins contains a region of homology that has the potential to form a helix-loop-helix (HLH) structure preceded by a highly basic region (22); this region has been shown to contain a DNA-binding and dimerization domain (12, 22, 29). Certain HLH homo- and heterodimers bind to a common DNA sequence, termed the E2-box. The E2-box is present in the immunoglobulin heavyan
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