Members of the Id family of basic helix-loop-helix (bHLH) proteins include Id1, Id2, Id3 and Id4. They are ubiquitously expressed and dimerize with members of the class A and B HLH proteins. Id2 interacts with a wide variety of transcription factors including E proteins, TCS, Pax and the tumor suppressor Rb. In tumor cells, increased levels of Id2 functionally inactivate Rb, leading to cellular transformation and cancer. ID-2 may be an inhibitor of tissue-specific gene expression.
