Professor Greg Hannon (1964 - 2026) was internationally recognised for his contributions to small RNA biology, cancer biology and mammalian genomics. He had a long history of discovering cancer genes and developed widely used tools and strategies for manipulating gene expression in mammalian cells and animals, including generating genome-wide shRNA libraries that were made available to the cancer research community. Greg was an extraordinary scientist, leader and collaborator. He was also a champion of Cancer Grand Challenges from its inception.