A 66-year-old man in California has been cured of HIV after he received a stem cell donation for leukemia from a donor who was naturally resistant to the virus.

The patient is known as the “City of Hope” patient because he refused to be identified after he was cured, Reuters reported. The patient is the fourth known patient to be cured this way.

The patient was diagnosed with the disease in 1988 but managed to control it for more than 30 years with antiretroviral therapy (ART).

Sharon Lewin, president-elect of the International Aids Society, said that the cure was the “holy grail” and that the story gives “continued hope … and inspiration” for those battling the disease.

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The report said that scientists believe that the treatment worked to cure the patient because the stem cell donor had a rare biogenetic makeup where they did not have the receptors needed to be infected by HIV.

Doctors said that they have found no signs of HIV in the man after he stopped antiretroviral therapy (ART) more than a year ago.

Read more: Daily Wire