Researchers sequenced the complete genome of a wild button mushroom strain (ARP23) that has been successfully bred with commercial mushrooms to create the ‘Heirloom’ variety. This strain is larger and contains more genes than other known button mushroom strains, making it valuable for breeding disease-resistant varieties. The genome sequence reveals that all button mushroom strains share core genes for breaking down plant material in compost, but have diverse collections of optional genes. This genetic resource provides a foundation for developing mushrooms more resistant to diseases and viruses.