Gregory J. Hannon passed away in April 2026 at the age of 61. A towering figure in modern molecular biology, Greg influenced remarkably diverse areas of science. His work reshaped our understanding of gene regulation, advanced the discovery of tumor suppressors that regulate the cell-cycle machinery, and uncovered fundamental mechanisms of RNA interference. By establishing core principles of small RNA biology, he not only transformed our mechanistic view of RNA-silencing pathways but also pioneered functional genomics approaches that revolutionized basic and biomedical research in mammalian systems.