We have shown that selective inhibitors of dimer formation between the oncogenic basic-helix-loop-helix-leucine zipper (bHLHZip) trancription factor c Myc and its bHLHZip partner protein Max act by binding to the intrinsically disordered (ID) c Myc monomer. Multiple, independent sites for inhibitor binding were found along c-Myc bHLHZip. We exploited the multiplicity of these sites to generate novel compounds capable of multivalent binding to c-Myc. Despite their disordered protein target, these molecules bind purified c-Myc with low nano-molar affinity, which is orders of magnitude tighter than that of c-Myc's obligate heterodimerization partner Max. The inhibitors effectively disrupt c-Myc-Max dimerization and specific DNA binding; they also inhibit growth of c-Myc overexpressing cancer cell lines in vitro.