Wojcicki, who was 56, stepped down from the CEO job at YouTube last year after more than two decades leading various parts of Google and its parent company Alphabet.

“Even as I write this it feels impossible to me that it’s true,” Alphabet CEO Sundar Pichai wrote in a note to staffers on Friday that was published on the company website.

“Her loss is devastating for all of us who know and love her, for the thousands of Googlers she led over the years, and for millions of people all over the world who looked up to her, benefited from her advocacy and leadership, and felt the impact of the incredible things she created at Google, YouTube, and beyond.”

In a post on X on Saturday, Apple CEO Tim Cook wrote, “Saddened to hear of the passing of Susan Wojcicki. She was one of Silicon Valley’s visionaries and she will be missed by so many. May she rest in peace.”

Wojcicki was a key figure at Google from the company’s earliest days, when she rented her Palo Alto garage to founders Larry Page and Sergey Brin. She soon quit her job at chipmaker Intel and joined the fledgling search startup, becoming its first marketing manager in 1999. Over the years, she would oversee Google’s advertising business and its video business, playing a major role in the company’s transformation from startup into today’s $2 trillion tech juggernaut.

The Wojcicki family became known as something close to Silicon Valley royalty. Wojcicki’s sister Anne, the founder and CEO of genetics testing company 23andMe, was once married to Google cofounder Brin. Her mother, Esther, founded the journalism program at Palo Alto High School, in the cradle of the tech industry, and was recognized in a 2012 Digital Learning Day by then U.S. Education Secretary Arne Duncan for using technology effectively in the classroom.

Wojcicki’s husband, Dennis Troper, who is a director of product managment at Google, wrote in a Facebook post on Friday that Wojcicki had been living with “non small cell lung cancer” for two years. “Susan was not just my best friend and partner in life, but a brilliant mind, a loving mother, and a dear friend to many,” Troper wrote.

As YouTube’s boss, Wojcicki oversaw one of the world’s largest media sites, with an audience that streams more than 1 billion hours of video every day. The video site, which Wojcicki pushed Google to acquire in 2006, generated $8.7 billon in advertising revenue in the second quarter. During the nine years in which Wojcicki led YouTube as CEO, she turned the video site into a reliable business amid a changing market while also battling — not always successfully — the rampant spread of disinformation.

In an interview with Fortune Editor-in-Chief Alyson Shontell at the 2022 World Economic Forum in Davos, Wojcicki discussed the challenges of regulating the content it publishes on the site and that it recommends to users.

“If you are dealing with a sensitive subject like news, health, science, we are going to make sure that what we’re recommending is coming from a trusted, well-known publisher that can be reliable,” she said.

She also addressed important issues involving women in the workplace, including the U.S. Supreme Court’s ruling that overturned the Roe vs. Wade decision guaranteeing women the right to abortion.

“My stance is that women should have a choice when they become a mother. I believe that’s really important. I believe that reproductive rights are human rights and to take away a law and a right that we’ve had for almost 50 years will be a big setback for women. But that’s my personal view,” she said. “Running a company that really focuses on free speech, we want to make sure that we’re enabling a broad set of opinions that everyone has a right to express their point of view, provided they meet our community guidelines.”

Read More about Susan Wojcicki:

• Susan Wojcicki perfectly predicted the media world we now live in—twice
• Susan Wojcicki on how she built YouTube as the new TV
• Susan Wojcicki was the most level-headed of her sisters while growing up, mother recalled in earlier talk about how to raise successful kids
• Susan Wojcicki offered management lessons from the Titanic and ‘Frozen’ in a 2014 commencement speech