Complex sentence: E-box binding proteins usually contains the basic helix-loop-helix protein structural motif which allows them to bind as dimers Ellenberger T, Fass D, Arnaud M, Harrison S C. "Genes Dev".
Split into simpler sentences: E-box binding proteins usually contains the basic helix-loop-helix protein structural motif. The HLH motif within the bHLH protein allows it to bind to the E-box as a dimer Ellenberger T, Fass D, Arnaud M, Harrison S C. "Genes Dev".