The phenotypic modulation of vascular smooth muscle cells (VSMCs) from a quiescent, differentiated state to that of proliferating, """"""""undifferentiated"""""""" cells is a common pathogenic feature of vascular disease. The molecular mechanisms by which this phenotypic """"""""switch"""""""" is achieved are unknown. We have observed that proliferating vascular smooth muscle cells express mRNAs that are homologous to a gene expressed in other tissues that acts as a transdominant suppressor of differentiation. This gene is referred to as ID (inhibitor of differentiation). It belongs to the family of helix-loop-helix (HLH) family of proteins, many of which are involved in cell determination and differentiation. Using a variety of molecular cloning techniques, we have further identified and cloned two ID-like cDNAs from proliferating VSMCs. One of these is the rat homologue of mouse ID1 (i.e. rat ID1) and the other is apparently a unique member of the ID family. We have further demonstrated that rat ID1 exists as two protein isoforms which are the products of alternative RNA splicing of the rat ID1 gene. Expression of rat ID1 is upregulated by a number of growth factors that are important in initiating and sustaining VSMC proliferation, such as PDGF, bFGF, and IGF-1. Expression of rat ID1 is often (but not always) downregulated when VSMCs are made quiescent. However, when cultured VSMCs achieve a level of differentiation comparable to that in the intact vessel (for example, when they are cultured on reconstituted basement membrane), ID expression decreases 10-fold to a level similar to that seen in the intact, uninjured vessel. Our group is currently initiating studies to identify the function of ID in VSMC migration, proliferation, and differentiation by overexpressing rat ID1 from constitutively active and inducible promoters, by antisense inhibition of rat ID1, and by genetic screening of cDNA libraries using the two hybrid yeast system to identify potential protein partners for rat ID1.