Microsoft co-founder Paul Allen died today from complications with non-Hodgkin’s lymphoma. He was 65. Allen said earlier this month that he was being treated for the disease.

Allen was a childhood friend of Bill Gates, and together, the two started Microsoft in 1975. He left the company in 1983 while being treated for Hodgkin’s lymphoma and remained a board member with the company through 2000. He was first treated for non-Hodgkin’s lymphoma in 2009, before seeing it go into remission.

In a statement given to ABC News, Gates said he was “heartbroken by the passing of one of my oldest and dearest friends.” He went on to commend his fellow co-founder for his life after Microsoft:

From our early days together at Lakeside School, through our partnership in the creation of Microsoft, to some of our joint philanthropic projects over the years, Paul was a true partner and dear friend. Personal computing would not have existed without him. But Paul wasn’t content with starting one company. He channelled his intellect and compassion into a second act focused on improving people’s lives and strengthening communities in Seattle and around the world. He was fond of saying, “If it has the potential to do good, then we should do it.” That’s the king of person he was. Paul loved life and those around him, and we all cherished him in return. He deserved much more time, but his contributions to the world of technology and philanthropy will live on for generations to come. I will miss him tremendously.

Microsoft CEO Satya Nadella said Allen’s contributions to both Microsoft and the industry were “indispensable.” His full statement is quoted below:

Paul Allen’s contributions to our company, our industry, and to our community are indispensable. As co-founder of Microsoft, in his own quiet and persistent way, he created magical products, experiences and institutions, and in doing so, he changed the world. I have learned so much from him — his inquisitiveness, curiosity, and push for high standards is something that will continue to inspire me and all of us as Microsoft. Our hearts are with Paul’s family and loved ones. Rest in peace.

In a memoir published in 2011, Allen says that he was responsible for naming Microsoft and creating the two-button mouse. The book also portrayed Allen as going under-credited for his work at Microsoft, and Gates as having taken more ownership of the company than he deserved. It created some drama when it arrived, but the two men ultimately appeared to remain friends, posing for a photo together two years later.

After leaving Microsoft, Allen became an investor through his company Vulcan, buying into a diverse set of companies and markets. Vulcan’s current portfolio ranges from the Museum of Pop Culture in Seattle, to a group focused on using machine learning for climate preservation, to Stratolaunch, which is creating a spaceplane. Allen’s investments and donations made him a major name in Seattle, where much of his work was focused. He recently funded a $46 million building in South Seattle that will house homeless and low-income families.

Both Apple CEO Tim Cook and Google CEO Sundar Pichai called Allen a tech “pioneer” while highlighting his philanthropic work in statements on Twitter. Amazon CEO Jeff Bezos said Allen’s work “inspired so many.”

Our industry has lost a pioneer and our world has lost a force for good. We send our deepest condolences to Paul’s friends, the Allen family and everyone at Microsoft. — Tim Cook (@tim_cook) October 15, 2018

We lost a great technology pioneer today - thank you Paul Allen for your immense contributions to the world through your work and your philanthropy. Thoughts are with his family and the entire Microsoft community. — Sundar Pichai (@sundarpichai) October 15, 2018

Very sad to hear of Paul Allen’s passing. His passion for invention and pushing forward inspired so many. He was relentless to the end. My heart goes out to Paul’s family and friends. — Jeff Bezos (@JeffBezos) October 15, 2018

Allen has long been the owner of the Portland Trail Blazers and Seattle Seahawks as well. NFL Commissioner Roger Goodell said Allen “worked tirelessly” to “identify new ways to make the game safer and protect our players from unnecessary risk.” NBA Commissioner Adam Silver said Allen “helped lay the foundation for the league’s growth internationally and our embrace of new technologies.”

He also launched a number of philanthropic efforts, which were later combined under the name Paul G. Allen Philanthropies. His “philanthropic contributions exceed $2 billion,” according to Allen’s own website, and he had committed to giving away the majority of his fortune.

Allen’s sister, Jody Allen, wrote a statement on his family’s behalf:

My brother was a remarkable individual on every level. While most knew Paul Allen as a technologist and philanthropist, for us he was a much loved brother and uncle, and an exceptional friend. Paul’s family and friends were blessed to experience his wit, warmth, his generosity and deep concern. For all the demands on his schedule, there was always time for family and friends. At this time of loss and grief for us – and so many others – we are profoundly grateful for the care and concern he demonstrated every day.

Some of Allen’s philanthropy has taken a scientific bent: Allen founded the Allen Institute for Brain Science in 2003, pouring $500 million into the non-profit that aims to give scientists the tools and data they need to probe how brain works. One recent project, the Allen Brain Observatory, provides an open-access “catalogue of activity in the mouse’s brain,” Saskia de Vries, senior scientist on the project, said in a video. That kind of data is key to piecing together how the brain processes information.

In an interview with Matthew Herper at Forbes, Allen called the brain “hideously complex” — much more so than a computer. “As an ex-programmer I’m still just curious about how the brain functions, how that flow of information really happens,” he said. After founding the brain science institute, Allen also founded the Allen Institute for Artificial Intelligence and the Allen Institute for Cell Science in 2014, as well as the Paul G. Allen Frontiers Group in 2016, which funds cutting-edge research.

Even back in 2012, when Allen spoke with Herper at Forbes, he talked about plans for his financial legacy after his death — and he said that a large part of it would be “allocated to this kind of work for the future.”

In a statement emailed to The Verge, The Allen Institute’s President and CEO Allan Jones said:

Paul’s vision and insight have been an inspiration to me and to many others both here at the Institute that bears his name, and in the myriad of other areas that made up the fantastic universe of his interests. He will be sorely missed. We honor his legacy today, and every day into the long future of the Allen Institute, by carrying out our mission of tackling the hard problems in bioscience and making a significant difference in our respective fields.

According to Quincy Jones, Allen was also an excellent guitar player.

Update 10/15, 8:46PM ET: Added statement from Bill Gates.