Id proteins (inhibitors of DNA binding/differentiation) are negative regula
tors of basic helix-loop-helix (bHLH) type transcription factors, which pro
mote the differentiation of various cell types. In addition to their "class
ical" ability to inhibit cell differentiation, they are able to stimulate c
ell cycle progression. These facts suggest that Id proteins play a role in
keeping precursor cells immature and in expanding the cell population size
during development. In vitro as well as in vivo analyses in the last severa
l years have shown that Id proteins have more complex activities; they indu
ce apoptosis or function as survival factors, depending on the cell context
. Furthermore, dysregulated expression of Id proteins has been reported in
several human tumors and seems to be related to the malignant character of
tumors. Here, we summarize and discuss the biological activities of Id prot
eins from the standpoint of cell growth control. (C) 2002 Wiley-Liss, Inc.