ID proteins contain a helix-loop-helix (HLH) motif and regulate tissue-specific transcription within several cell lineages. They do not bind DNA directly, but inhibit lineage commitment by binding basic helix-loop-helix (bHLH) transcription factors through their HLH motif. ID proteins contribute to cell growth, senescence, differentiation and angiogenesis. Id1 mRNA is highly expressed in heart, lung and kidney and has lower expression in brain and liver. Two transcript variants encoding different isoforms have been found for this gene.