The fight to beat HIV has been going on for a long, long time. The disease, which can be treated but not yet reliably beaten, often wins, but news out of London suggests that researchers may beginning to turn the tide. In what would only be the second time in medical history, an HIV patient appears to have been cured of the disease thanks to a bone marrow stem cell transplant.

The man, who has been HIV positive since at least 2003, now appears to have had the virus driven from his system by a very special genetic mutation present in the stem cells of a donor.

The breakthrough, which is detailed in a new paper in Nature, utilized the natural HIV-fighting power of a mutation called “CCR5 delta 32” which makes certain people resistant to the virus. A transplant of bone marrow stem cells from a donor with that specific mutation has seemingly cured the man, known only as the “London patient,” of his HIV infection.

Testing over the past 18 months have shown no traces of HIV in the man’s system, and the case matches that of the first documented case of HIV being cured back in 2007. That man, Timothy Ray Brown (known only as the “Berlin patient” at the time), received a similar bone marrow transplant which cured him of the disease.

This latest development is obviously big news for researchers hoping to cure HIV on a larger scale, but this particular case was extremely unique. The London patient ultimately had no option but to try the experimental treatment when he was diagnosed with Hodgkin’s lymphoma in 2012. Chemotherapy plus the stem cell transplants were what lead researcher Ravindra Gupta called a “last chance at survival” for the patient.

The mutation that gives a person HIV resistance is incredibly rare, only appearing in a tiny fraction of the population. Treating an HIV patient with bone marrow stem cells from someone with the beneficial mutation means finding a person that matches the recipient’s biology and also happens to have the rare mutation.

However, now that science has determined that the earlier Berlin patient’s HIV cure wasn’t merely a strange fluke, it could open up the doors to new gene-level treatments for the disease.