Timothy Ray Brown was diagnosed with HIV in 1995 and began antiretroviral therapy. In 2006, Timothy was diagnosed with acute myeloid leukemia (AML). His physician, Dr. Gero Hütter, at Charité Hospital in Berlin, arranged for him to receive a hematopoietic stem cell transplant from a donor with the CCR5 delta32 mutation.This mutation, found at relatively high frequencies in Northern Europeans, results in a mutated CCR5 protein.The majority of HIV cannot enter a human cell without a functional CCR5 gene. An exception to this is a small minority of viruses that use alternate receptors, such as CXCR4 or CCR2.Those individuals who are homozygous for the CCR5 mutation are resistant to HIV and rarely progress to AIDS. Timothy received two stem cell transplants from one donor homozygous for the delta32 mutation: one in 2007 and one in 2008. In 2009, after being off antiretroviral therapy for one year, levels of HIV plummeted while his CD4 T cell count increased, indicating that he no longer needed antiretroviral therapy to control his HIV infection.