Id helix—loop—helix proteins in cell growth and differentiation

Id helix-loop-helix proteins function at a general level as positive regulators of cell growth and as negative regulators of cell differentiation. They act as dominant-negative antagonists of other helix-loop-helix transcription factors, which drive cell lineage commitment and differentiation in diverse cell types of higher eukaryotes. In addition, the functions of Id proteins are integrated with cell-cycle-regulatory pathways orchestrated by cyclin-dependent kinases and the retinoblastoma protein. Here, some of the recent advances that highlight the importance of Id proteins as regulatory intermediates for coordinating differentiation-linked gene expression with cell-cycle control in response to extracellular signalling are reviewed.