Id proteins were originally characterized as inhibitors of DNA binding and cell differentiation. Id1 through 4 contain an evolutionarily conserved helix-loop-helix (HLH) sequence which is critical for protein-protein interaction(s). Most HLH transcription factors contain a basic amino acid region adjacent to the HLH sequence, the bHLH sequence, which is responsible for DNA binding. bHLH transcription factors fall into 2 major groups designated class A factors, e.g.,E2.2 and E47, and class B factors, e.g., MyoD and myogenin. In vitro studies demonstrate distinct interaction(s) between Id proteins and bHLH transcription factors. While Id proteins contain an HLH domain, they lack the basic region which is required for DNA binding. Therefore, Id proteins are negative regulators of transcription since complexes which contain them do not bind DNA. Id proteins are variably expressed throughout the cell cycle and are regulated by phosphorylation by cyclin-cdk complexes. Thus, Id proteins play an important role in transcriptional regulation of cell cycle related genes. Overexpression of Id3 can induce apoptosis in serum-starved fibroblasts by a mechanism which is correlated to Id3-mediated cell cycle progression and is rescued by over expression of the antiapoptotic proteins Bcl-2 and Bcl-XL. Clone B72-1 recognizes mouse Id3. It does not cross react with mouse Id1 or mouse Id2. A fusion protein containing full length mouse Id3 was used as immunogen. The antibody is routinely tested by western blotting of the Id3-fusion protein. The Id3-fusion protein is observed at 43 kDa (fusion protein MW ~28 kDa and Id3 MW is ~13 kDa).