WOW: Second man seems to be free of AIDS virus after transplant.

The patient has not been identified. He was diagnosed with HIV in 2003 and started taking drugs to control the infection in 2012. It’s unclear why he waited that long. He developed Hodgkin lymphoma that year and agreed to a stem cell transplant to treat the cancer in 2016.

With the right kind of donor, his doctors figured, the London patient might get a bonus beyond treating his cancer: a possible HIV cure.

Doctors found a donor with a gene mutation that confers natural resistance to HIV. About 1 percent of people descended from northern Europeans have inherited the mutation from both parents and are immune to most HIV. The donor had this double copy of the mutation.

That was “an improbable event,” said lead researcher Ravindra Gupta of University College London. “That’s why this has not been observed more frequently.”

The transplant changed the London patient’s immune system, giving him the donor’s mutation and HIV resistance.

The patient voluntarily stopped taking HIV drugs to see if the virus would come back.

Usually, HIV patients expect to stay on daily pills for life to suppress the virus. When drugs are stopped, the virus roars back, usually in two to three weeks.

That didn’t happen with the London patient. There is still no trace of the virus after 18 months off the drugs.

Next up: Developing a technique to grow stem cells with that mutation in a lab?