William Kaelin is an American cancer researcher known for his work on the role of oxygen in cellular processes. He was awarded the Nobel Prize in Physiology or Medicine in 2019, along with Sir Peter Ratcliffe and Gregg Semenza, for their discoveries related to how cells sense and adapt to oxygen availability. Kaelin's research has significant implications for understanding cancer, as tumors often exploit these oxygen-sensing pathways to grow and survive. He is a professor at Harvard Medical School and has contributed to advancements in cancer treatment and therapies through his scientific findings.