Microsoft Co-Founder Paul Allen died from complications of non-Hodgkin's lymphoma on Monday afternoon.
Allen's Vulcan Inc. announced that he died in Seattle at 65 years old.
Allen's sister, Jody, said he 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," she said in a statement. "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."
Allen ranked among the world's wealthiest individuals. As of Monday afternoon, he ranked 44th on Forbes' 2018 list of billionaires with an estimated net worth of more than $20 billion.
Through Vulcan, Allen's network of philanthropic efforts and organizations, the Microsoft co-founder supported research in artificial intelligence and new frontier technologies. The group also invested in Seattle's cultural institutions and the revitalization of parts of the city.
Allen owned two professional sports teams, the NFL Seattle Seahawks and NBA Portland Trailblazers. He was also an electric guitarist who occasionally jammed with celebrity musicians including Bono and Mick Jagger, and a huge music fan. He funded and designed the Experience Music Project in Seattle, devoted to the history of rock music and dedicated to his musical hero Jimi Hendrix. (It has since been re-christened the Museum of Pop Culture.) The building was designed by architect Frank Gehry to resemble a melted electric guitar.
Vulcan CEO Bill Hilf said, "All of us who had the honor of working with Paul feel inexpressible loss today."
"He possessed a remarkable intellect and a passion to solve some of the world's most difficult problems, with the conviction that creative thinking and new approaches could make profound and lasting impact," Hilf said in a statement.
Earlier this month, Allen revealed that he had started treatment for non-Hodgkin's lymphoma, the same type of cancer he was treated for in 2009. In 1983, Allen left the company he founded with Bill Gates when he was first diagnosed with Hodgkin's disease, which he defeated.
Bill Gates, who co-founded Microsoft with Allen, said that "personal computing would not have existed without him":
"I am heartbroken by the passing of one of my oldest and dearest friends, Paul Allen. 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 channeled 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 kind 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."
Current Microsoft CEO Satya Nadella said Allen made "indispensible" contributions to Microsoft and the technology industry. Nadella also said he learned a lot from Allen and will continue to be inspired by him.
"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," Nadella said in a statement.
Former Microsoft CEO Steve Ballmer called Allen a "truly wonderful, bright and inspiring person."
Steven Sinofsky, former president of Microsoft's Windows division, said Allen "did so much to shape lives with computing and his later work in science, community, and research."
Seahawks Coach Pete Carroll said he was deeply saddened by Allen's death.
NFL Commissioner Roger Goodell said Allen was "the driving force behind keeping the NFL in the Pacific Northwest." Goodell said he valued Allen's advice on a wide range of subjects and sent his condolences.
"His passion for the game, combined with his quiet determination, led to a model organization on and off the field. He worked tirelessly alongside our medical advisers to identify new ways to make the game safer and protect our players from unnecessary risk" Goodell said in a statement.
The Trail Blazers tweeted, "We miss you. We thank you. We love you."
Allen's death was met with an outpouring of condolences from tech leaders. Google CEO Sundar Pichai said with Allen's death, the world has "lost a great technology pioneer today."
Apple CEO Tim Cook called him a "pioneer" and a "force for good."
Salesforce CEO Marc Benioff said he was saddened by Allen's passing.
Amazon CEO Jeff Bezos praised his "relentless" push forward in technology:
— CNBC's Matt Rosoff, Ryan Ruggiero and Reuters contributed to this report.
Clarification: This story has been updated to reflect that Microsoft Co-Founder Paul Allen left the company after being diagnosed with Hodgkin's disease. A previous version mischaracterized the timeline of his departure.