Paul Allen, who founded Microsoft alongside Bill Gates in 1975, has died at the age of 65.
Mr Allen, who suffered from non-Hodgkin lymphoma, was also a prominent philanthropist, donating billions to science, conservation and the arts.
Eight years after starting the company he received his first cancer diagnosis, but it was treated successfully, though it forced him to retire early from his role at Microsoft. He developed non-Hodgkin lymphoma in 2009, which had been in remission until it returned earlier this year.
In a statement released at the start of this month, Mr Allen said he planned to fight the disease "aggressively".
“A lot has happened in medicine since I overcame this disease in 2009. My doctors are optimistic that I will see good results from the latest therapies, as am I," he said.
On Monday his sister Jody Allen called him 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.
"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," she said.
Mr Gates led the tributes from the tech world, remembering Mr Allen as "one of my oldest and dearest friends."
"Personal computing would not have existed without him," Mr Gates added.
"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."
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
Current Microsoft CEO Satya Nadella said his contributions to technology had been "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," he said.
Tim Cook, CEO of Apple, said the tech industry had "lost a pioneer and our world has lost a force for good".
Mr Allen never married and had no children, and was sometimes described as reclusive.
His early exit from Microsoft meant he dedicated much of his life to scientific and technological research, including the foundation of the Allen Institute, which began by focusing on neuroscience but has widened its approach to cover bioscience, and a more recent project to build a reusable space plane, designed to launch satellites.
So sad to hear about the passing of Paul Allen. Among many other things he was a pioneer of commercial space travel. We shared a belief that by exploring space in new ways we can improve life on Earth. All our thoughts are with his loved ones.— Richard Branson (@richardbranson) October 15, 2018
He also owned American football team the Seattle Seahawks and basketball team the Portland Trailblazers.
In a joint statement the organisations paid tribute to Mr Allen.
On their behalf Bill Hilf, CEO of Vulcan, which oversaw his businesses, said: "All of us who had the honour of working with Paul feel inexpressible loss today.
Sad to hear of the passing of Paul Allen, who was a strong advocate for environmental protection. He and the team at Vulcan played a pivotal role in developing the Shark Conservation Fund alongside LDF. His legacy lives on via his incredible work as a philanthropist and investor.— Leonardo DiCaprio (@LeoDiCaprio) October 16, 2018
"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.
"Millions of people were touched by his generosity, his persistence in pursuit of a better world, and his drive to accomplish as much as he could with the time and resources at his disposal."