Basic helix-loop-helix proteins (bHLH) are a group of eukaryotic transcription factors that exert a determinative influence in a variety of developmental pathways. These transcription factors are characterised by a highly evolutionary conserved bHLH domain that mediates specific dimerisation [<cite idref="PUB00004974"/>]. They facilitate the conversion of inactive monomers to trans-activating dimers at appropriate stages of development [<cite idref="PUB00000717"/>]. <p> The bHLH proteins can be classified into discrete categories. One such subdivision according to dimerisation, DNA binding and expression characteristics defines seven groups [<cite idref="PUB00007119"/>]. Class I proteins form dimers within the group or with class II proteins. Class II can only form heterodimers with class I factors. Class III factors are characterised by the presence of a leucine zipper (<db_xref db="INTERPRO" dbkey="IPR003327"/>) adjacent to the bHLH domain. Class IV factors may form homodimers or teterodimers with class III proteins. Class V and class VI proteins act as regulators of class I and class II factors and class VII proteins have a PAS domain (<db_xref db="INTERPRO" dbkey="IPR000014"/>).</p>