Timothy Ray Brown, an American working in Berlin as a translator, was diagnosed with HIV and later with leukemia. To treat his cancer, his physician in Berlin, Gero Hütter, gave him a stem cell transplant from a donor who carried a mutation in the CCR5 protein known as delta32. For years he was known in medical reports only as the Berlin patient.

Brown's first transplant, in 2007, was only partly successful: his HIV appeared to be gone, but his leukemia was not. In 2008 he received a second transplant from the same donor, and it appeared to cure both diseases. The results were published in The New England Journal of Medicine. Brown became the first person known to be cured of HIV, at a time when many scientists doubted that it could be done. Hütter later said that Brown showed it was possible, under special circumstances, to rid a patient of the virus.

The treatment was not a practical path for most people. Brown's…