Three important classes in the bHLH family, which are the basis for the mathematical model presented below, are the class A, ubiquitously-expressed transcriptional activators capable of forming homodimers and heterodimers, the class B, capable of providing promoter-specific transcriptional activation only when heterodimerized with a class A element, and Id proteins, which have been most often reported to form transcriptionally-unproductive heterodimers with the class A. Since different class B proteins bind the same class A partners, there can be some competition between them for access to those partners. In networks in which class B proteins auto-activate their own transcription (a common feature of determinants of cellular differentiation), class B proteins can therefore inhibit one another's expression, by titrating out the class A. Id proteins have the same effect of titrating out the class A, but are not explicitly taken into account in the model below because they have not been shown to regulate their own expression.