Y Combinator president Sam Altman is stepping down amid a series of changes at the accelerator

Sam Altman, the well-known president of the prolific Silicon Valley accelerator Y Combinator, is stepping down, the firm shared in a blog post on Friday.

Altman is transitioning into a chairman role with other YC partners stepping up to take on his day-to-day responsibilities, as first reported by Axios. Sources tell TechCrunch YC has no succession plans. YC’s core program is currently led by chief executive officer Michael Seibel, who joined the firm as a part-time partner in 2013 and assumed the top role in 2016.

The news comes amid a series of shake-ups at the accelerator, which is expected to demo its latest batch of 200-plus companies in San Francisco March 18 and 19. In Friday’s blog post, YC expands on some of those changes, including the firm’s decision to move it’s HQ to San Francisco, which TechCrunch reported earlier this week.

“We are considering moving YC to the city and are currently looking for space,” YC writes. “The center of gravity for new startups has clearly shifted over the past five years, and although we love our space in Mountain View, we are rethinking whether the logistical tradeoff is worth it, especially given how difficult the commute has become. We also want to be closer to our Bay Area alumni, who disproportionately live and work in San Francisco.”

In addition to moving it’s HQ up north, YC has greatly expanded the size of its cohorts — so much so that it’s next demo day will have two stages — and it’s writing larger checks to portfolio companies.

Altman, who joined YC as a partner in 2011 and was named president in 2014, will focus on other efforts, including OpenAI, a research organization in which he co-chairs. Altman was the second-ever YC president, succeeding YC co-founder Paul Graham in 2014. Graham is currently an advisor to YC.