As NM 3T3 fibrobiasts become quiescent, the level of c&n protein m while JunD accumulates. When resting ceils are stimulated with fresh serum, m&ear-iocallxed JunD is rapldty degraded, followed by resyntheals of both cJ &in and JunD later In Gl. Dverexpre&on of JunD results In dower growth and an lncreaee In the percentage of ceils in oO/Gl while c-Jun overexpmselon produces larger S/G2 and M pha8e pop&Dons. In addition, JunD partially suppresses transformation by an activated ras gene whereas c-Jun cooperates with ras to transform cells. These data indicate that two closely related tranacrip tion factors can function in an opposing manner.