cMyc is a proto-oncogene that codes for an important transcription factor1. It is overexpressed and constitutively active in many aggressive human cancers, tumourigenic in mice, and its inhibition causes tumour regression with minimal side effects1. However, cMyc is an intrinsically disordered protein until bound by its partner protein Max in a coiled coil protein-protein complex. Its binding makes many contacts across large and shallow interacting surfaces that have been difficult to inhibit with small drug-like molecules1. Here we downsize Max to short fragment peptides, and fix and optimise them in Myc-binding conformations that impart selectivity. We also incorporate different electrophilic warheads2,3, and demonstrate efficient covalent binding to a residue in cMyc that is unique among Myc/Max/Mad/Mga/Mnt proteins. Irreversible capture can potentially silence transcriptional activation by Myc, leaving Max/Mad/Mga/Mnt free to heterodimerise and form transcriptional repressor complexes. Activity of cMyc would therefore only be possible through its resynthesis from the ribosome. These studies on novel constrained peptides that can compete with endogenous ligands for binding to cMyc suggest a possible new route to cancer therapies.