Membrane fusion proteins (not to be confused with chimeric or fusion proteins) are proteins that cause fusion of biological membranes. Membrane fusion is critical for many biological processes, especially in eukaryotic development and viral entry. Fusion proteins can originate from genes encoded by infectious enveloped viruses, ancient retroviruses integrated into the host genome,[1] or solely by the host genome.[2] Post-transcriptional modifications made to the fusion proteins by the host, namely addition and modification of glycans and acetyl groups, can drastically affect fusogenicity (the ability to fuse).[3]