Members of the myogenic determination family are basic helix-loop-helix (bHLH) proteins that can be separated into two classes. Class A proteins include the ubiquitously expressed E-box binding factors E12/E47, ITF2 and HEB (BETA 1 or HTF4). Class B proteins such as Myo D, myogenin and Neuro D (BETA 2) are transiently expressed and exhibit a more limited tissue distribution. Class A proteins heterodimerize with class B proteins to activate transcription. Working in opposition to these positively acting factors are a specialized group of proteins that function as dominant negative regulators. For instance, the Id family of transcriptional repressors contains a HLH region required for dimerization but lacks a functional DNA-binding domain. The Id family can therefore form heterodimers with the myogenic family, but the resulting complexes are transcriptionally inactive. BETA 3 is a protein that is functionally similar to members of the Id family in that it can inhibit the binding of E47 homodimers as well as E47/Neuro D and E47/Myo D heterodimers to consensus DNA sequences . In contrast to members of the Id family, BETA 3 contains a putative DNA-binding domain.