PROJECT SUMMARY MYC is an oncoprotein transcription factor that features prominently in cancer. As a transcription factor, the ability of MYC to recognize its target genes is paramount to its activity. The long-standing paradigm for how MYC selects its targets is that it dimerizes with MAX, forming a module that binds specific DNA sequences in the regulatory elements of target genes. In recent years, however, it has become clear that target gene recognition by MYC can also depend on additional chromatin-resident proteins that act through avidity to direct MYC:MAX dimers to specific sites in the genome. This 'facilitated recruitment' process is poorly understood, although it likely influences a majority of MYC binding to chromatin in cancer cells. Detailed mechanistic studies of facilitated recruitment are needed to understand this most basic aspect of MYC activity, and are timely because, unlike MYC, these recruiters may be amenable to drug discovery, unlocking new ways to target MYC in the clinic. This project explores the mechanisms and significance of facilitated recruitment of MYC to chromatin by WDR5, a conserved nuclear protein that is an active target for drug discovery by numerous groups. WDR5 recruits MYC to chromatin at genes vital for protein synthesis, including a collection of ribosomal protein genes and genes encoding translation factors and nucleolar RNAs. Disrupting interaction of MYC with WDR5 in a preexisting malignancy promotes rapid and irreversible tumor collapse, indicating that the MYC–WDR5 nexus can be pharmacologically inhibited as a treatment for MYC-driven cancers. Aim 1 of this project uses a combination of high resolution genetic, genomic, and proteomic approaches to characterize the fundamental mechanisms that bring MYC and WDR5 together on chromatin, and to reveal the extent to which facilitated recruitment by WDR5—and other recruiters—determines the genes that are controlled by MYC in cancer cells. Aim 2 blends genetic, genomic, and in vivo studies to probe the importance of the MYC–WDR5 connection in a diverse set of cancer contexts, and to reveal precisely how targeting this connection promotes tumor regression. These studies will lead to a new and robust paradigm for the mechanisms of target gene selection by MYC, identify novel and targetable vulnerabilities in the MYC network, and show how gene-selective recruiters such as WDR5 can be exploited to therapeutically inhibit MYC. Importantly, these studies will also lay the biological groundwork for the implementation of WDR5 inhibitors in the clinic as anti-MYC agents.