Allen died Monday at the age of 65, his investment company Vulcan announced in a tweet Monday. Allen, a survivor of Hodgkin's disease, was diagnosed with non-Hodgkin's lymphoma in 2009. Earlier this month, Allen announced on his website that the disease had returned but that he was going to fight it aggressively and was optimistic.
"I am very grateful for the support I've received from my family and friends," Allen wrote. "And I've appreciated the support of everyone on the teams and in the broader community in the past, and count on that support now as I fight this challenge."
Gates said in a statement he was "heartbroken by the passing of one of my oldest and dearest friends."
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.
Allen co-founded the software giant with Gates in 1975, but his partnership with Gates began in 1969, when Allen befriended the younger Gates at the private Seattle high school the two attended and began hanging out together in the computer room. The two honed their programming skills on time-sharing computer systems through the school's Teletype terminal.
Six years later, in Albuquerque, New Mexico, Allen and Gates began working on the programming language Microsoft BASIC after seeing the Altair 8800 -- what many consider to be the earliest desktop computer-- on the cover of the January 1975 issue of Popular Electronics.
The pair began calling their venture "Micro-Soft" -- a name Allen claimed credit for -- and by 1978, the young software maker had chalked up its first $1 million in sales. Microsoft struck the mother lode in 1981 when it licensed its IBM for use on its new personal computer. The company shrewdly retained the right to sell its software to other computer makers, setting the stage for a business model that made it the world's largest software maker.to
Allen left Microsoft in 1982 after being diagnosed with Hodgkin's disease, from which he recovered fully after several months of radiation therapy.
Allen became one of the richest men in the world when Microsoft went public in 1986, and he used his wealth to purchase the Portland Trail Blazers NBA basketball and Seattle Seahawks NFL football teams. In 1998, he bought a controlling interest in Charter Communications, now the second-largest cable TV provider in the US. He also founded Vulcan to make investments in emerging technologies and donate to philanthropic causes.
Allen was also the sole investor behind, a suborbital spacecraft that marked the first phase in the race to privatize space travel. In 2005, it reached a height of more than 100 kilometers -- generally considered the boundary of outer space -- over the Mojave desert.
In 2011, Allen announced the launch of Stratolaunch Systems, which would-- the world's largest -- to carry a rocket into the sky. The rocket would then disengage, fire its engines and head off toward space with the aim of reaching low Earth orbit.
After his departure from Microsoft, Allen Steve Ballmer (who would later become CEO) sought ways to take ownership stakes from Allen's share of Microsoft -- even when Allen was wrestling with cancer.with Gates following the publication of Allen's 2011 book Idea Man. In it, Allen alleged that Gates and the then recently hired
After the controversy, Allen appeared on 60 Minutes and said the book was , but instead was meant to serve as a record of what happened.
'An original and a dear person'
Reaction to Allen's death was swift and widespread.
Ballmer, in a tweet, called Allen "a truly wonderful, bright and inspiring person -- and a great friend. I will miss him."
Steven Sinofsky, the former president of Microsoft's Windows division, said in a tweet he was "deeply saddened" by news of Allen's passing. "Paul was an original and a dear person who did so much to shape lives with computing and his later work in science, community, and research," Sinofsky wrote.
Jeff Raikes, the former head of Microsoft's business division and former CEO of the Bill & Melinda Gates Foundation, called Allen an innovator with a big heart.
"Paul was an amazing intellect, quite innovative, as evidenced from his book, Idea Man," Raikes told CNET. "He had a lot of fabulous ideas. He also had a big heart. And one of the great things about his philanthropy was how he paired his intellect with things that were important to him, like economic mobility and saving elephants from poachers.
"He was an amazing human being," Raikes said. "The world is going to miss his contributions that came from his intellect and his heart."
In a statement, Microsoft CEO Satya Nadella said Allen's "contributions to our company, our industry and 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, his curiosity, and push for high standards is something that will continue to inspire me and all of us a Microsoft. Our hearts are with Paul's family and loved ones. Rest in peace."
NFL Commissioner Roger Goodall called Allen "the driving force behind keeping the NFL in the Pacific Northwest."
CNET's Ian Sherr and Rochelle Garner contributed to this report.
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