Prince Harry is honoring his late mother’s legacy in making a difference in the HIV community.

On Monday, which marked the special 40th anniversary of the Terrence Higgins Trust, the U.K.’s greatest HIV and sexual health charity released a letter from the Duke of Sussex.

“For the past 40 years, Terrence Higgins Trust has been revolutionizing the fight against HIV and AIDS. What began as honoring the loss of a loved one has evolved into the UK’s leading HIV and sexual health charity that provides essential education, funding and services to thousands living with HIV,” Harry noted in his letter, which was shared to Twitter.

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“I’ve been involved with Terrence Higgins Trust for a number of years, and the fight to end this epidemic is a big piece of my mum’s legacy. Like many, my mother grew up in a world where HIV was likely a death sentence,” the 38-year-old continued. “Yet, in the midst of all that uncertainty, she led with empathy, finding the humanity in all around her and demonstrating the power of connection in the face of fear. While my mother did not live to see the success of today’s treatments, I feel immense pride in being able to continue her advocacy with you.”

Harry, whose public work centers on HIV/AIDS, highlighted the organization’s ongoing aim- to “end new transmissions”- and pledged to assist England in becoming the first country to end new HIV diagnoses by 2030. He explained that the charity’s goals can be reached by encouraging testing, breaking stigmas surrounding silence and by donating to the trust.

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“My biggest and heartfelt congratulations on this historic milestone. May the next one we celebrate signal an end to the HIV/AIDS epidemic for all,” the Duke of Sussex concluded.

Harry’s letter was released hours before Terrence Higgins Trust’s annual fundraiser takes place later on Monday, which raises millions to support those affected by HIV and poor sexual health. The London event, The Auction at Christie’s, will open the catalog with his letter.

Prince Harry continues to fight against the stigma of HIV and Aids, following in Princess Diana’s footsteps. In 2017, shortly after his engagement to Meghan Markle was announced, he handed out self-testing kits at a pop-up shop with his now-wife at the Terrence Higgins Trust’s AIDS Day charity fair. Two years later, Harry teamed up with rugby player Gareth Thomas, who’d at the time revealed he was living with HIV, to promote National HIV Testing Week.

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Established in 1982, The Terrence Higgins Trust honors Terrence “Terry” Higgins, who was one of the first people to succumb to an AIDS-related illness in the U.K.

Furthering his efforts to end the stigma of HIV, Harry has also taken two HIV tests in public, one that was done with Rihanna in 2016 in light of World AIDS Day. He even created the charity Sentebale to help children affected by HIV and AIDS in Lesotho, Africa, where one of the world’s highest rates of HIV exists.