Edmond Fischer, who discovered the first example of enzyme regulation by reversible protein phosphorylation, died on August 27, 2021, aged 101. This fundamental discovery, which led to the realization that this process controls most aspects of cell life, earned Fischer and his colleague, friend, and collaborator Edwin Krebs, the 1992 Nobel Prize for Physiology or Medicine. Subsequently, other researchers developed drugs that switch off protein kinases, the enzymes that catalyse the covalent attachment of phosphate to proteins. More than 75 kinase-inhibiting drugs have been approved so far during the twenty-first century, which have transformed the clinical care and treatment of many cancers.