Figure 1.

The cofactor switch model. In the first scenario (upper part) Myc and cofactor X regulate the expression of numerous target genes, some of which may be transcription factors themselves (Target 1) that regulate their own target genes. The various direct or indirect targets then mediate a specific function (Function A) of c-Myc. In the second scenario (lower part) cofactor Y allows Myc to induce different target genes (α and β) and may block the ability of Myc to induce targets 1 and 2. As in the first scenario, some of the targets may be transcription factors that regulate their own targets that mediate Myc Function B indirectly. The different cofactors may be differentially expressed depending on conditions or cell type and may be competitive for Myc binding. The model also assumes that basal cofactors will be associated with Myc in both scenarios.