Rees is vice-president of discovery biology at AstraZeneca, with responsibility for reagent generation and assay development, functional genomics, and cell and gene therapy. Previously, he led the screening sciences department, with accountability for compound management, hit discovery and lead optimisation biology. Prior to joining AstraZeneca, he worked at GlaxoSmithKline for 24 years. He has served as chair of the European Laboratory Research and Innovation Group and chair of the European council of the Society for Laboratory Automation and Screening, and is industry trustee of the British Pharmacological Society. He was awarded an OBE in 2021 for services to science and the Covid-19 response.