Several observations suggest that the c-myc proto-oncogene is a key regulator of cell growth and differentiation. First, expression of c-myc RNA and protein is tightly regulated by mitogens and are suppressed by growth inhibitory agents (Luscher and Eisenman, 1991). Second, removal of c-myc protein appears required for withdrawal from the cell cycle since enforced c-myc expression promotes cell cycle progression (Askew et al., 1991; Eilers et al., 1991; Evan et al., 1992) and inhibits terminal differentiation (Prochownik and kukowska, 1986; Coppolla and Cole, 1986). Finally, inhibiton of c-myc expression by antisense oligonucleotides or RNA prevents entry of cells into S phase and accelerates differentiation (Heikkela et al, 1987; Prowchownik et al., 1988). The precise function that c-myc provides to promote cell cycle progression is unknown. Myc family proteins have domains characteristic of known transcription factors, including a basic-helix-loop-helix-(B-HLH) motif and a leucine zipper (LZ) domain, and have therefore been proposed to function as transcription factors (Luscher and Eisenman, 1991). Furthermore, it has recently been demonstrated that c-myc functions as a sequence-specific DNA binding protein (Blackwell et al., 1990; Prendergast and Ziff, 1991; Halazonetis and kandil, 1991) and through dimerization with a novel B-HLH-LZ partner, max, myc binds with high affinity to the “E-box” motif CACGTG (Blackwood and Eisenman, 1991). In addition, regions of the amino terminus of c-myc have been shown to function as transactivation domains when fused to the DNA binding domain of the transcription factor GAL4 (kato et al., 1990).