Id (inhibitors of DNA binding/differentiation) proteins, which inhibit cell differentiation and promote cell proliferation, are negative regulators of basic helix-loop-helix (bHLH) type transcription factor. There are four related members of the Id family called Id1, Id2, Id3 and Id4 in mammalian cells. Id proteins are critically involved in the regulation of cell cycle processes, including development, maturation, growth, differentiation, and apoptosis. Since the identification of Id proteins in 1990, the aspects about the roles of Id played in regulation of gene expression, cell proliferation and differentiation, senescence and tumorigenesis have been widely investigated. Thus, the Id proteins have become important molecules for understanding basic biological processes as well as targets for potential therapeutic intervention in human disease.