Id helix-loop-helix proteins function at a general level as positive r
egulators of cell growth and as negative regulators of cell differenti
ation. They act as dominant-negative antagonists of other helix-loop-h
elix transcription factors, which drive cell lineage commitment and di
fferentiation in diverse cell types of higher eukaryotes. In addition,
the functions of Id proteins are integrated with cell-cycle-regulator
y pathways orchestrated by cyclin-dependent kinases and the retinoblas
toma protein. Here, some of the recent advances that highlight the imp
ortance of Id proteins as regulatory intermediates for coordinating di
fferentiation-linken gene expression with cell-cycle control in respon
se to extracellular signalling are reviewed.