PROJECT SUMMARY/ABSTRACT Retrotransposons and retroviruses shape genome evolution and impact host fitness. Over 40% of the human genome is comprised of mobile DNA elements. These elements, once considered dormant, molecular fossils, are in fact remarkably influential across a diverse array of medically-relevant processes. Retroelements and domesticated components of mobile elements play important roles in neurodegeneration, cancer, pluripotent cells, innate immunity, and synaptic signaling. The retrovirus-like transposon Ty1 is an informative model for understanding mechanisms employed by retrotransposons and retroviruses across their replication cycles. This proposal leverages the genetic, biochemical, and cell biological toolkit developed for studying Ty1 and combines innovative conceptual frameworks from retrovirology, biophysics, protein biology, and cell biology. The proposed project will identify parameters governing Ty1 Gag’s role in virus-like particle assembly. This will advance our understanding of retroelement replication cycles and establish mechanisms that may be utilized by other disease-relevant retrotransposons, retroviruses, and domesticated capsid genes. In addition, identification of novel mechanisms used by retroelements has the potential to define a new class of antiviral drug targets.