Torsten Reil, the founder of Zynga‘s NaturalMotion division, has decided to leave the company for personal reasons.

Zynga just made the announcement, just after announcing its results for the first quarter. Reil has played a key role in the past couple of years at Zynga, running the United Kingdom-based studio that built the popular CSR Racing 2 game and just released Dawn of Titans five months ago.

During a different era under previous CEO Don Mattrick, Zynga paid $527 million to buy NaturalMotion. That deal helped Zynga expand beyond casual games to hardcore titles on mobile, such as racing and strategy games.

“We want to thank Torsten for his many contributions to Zynga over the last three years,” said Frank Gibeau, CEO of Zynga, in a statement. “Torsten built a great team and launched incredible franchises like CSR, Dawn of Titans and Clumsy Ninja. On a personal note, I’ve enjoyed working with Torsten in my time as a board member and CEO. We respect and support his decision to take a personal break after more than 15 years with NaturalMotion. We wish him all the best on his next chapter. Going forward, David Byrne, NaturalMotion’s COO, will step in to serve as interim leader of our NaturalMotion studio.”

It’s not clear what kind of change this means for Zynga or the NaturalMotion studio, but Zynga believes the transition will be smooth.

Zynga doesn’t disclose individual game revenues. But Dawn of Titans has grossed more than $21 million to date, with more than $10 million of that in the first quarter of 2017, according to measurement firm Sensor Tower. By comparison, CSR Racing 2 has grossed more than $60 million to date, with more than $15 million of that in Q1. The revenues for Dawn of Titans appears to be good, as analyst Michael Pachter of Wedbush Securities said in a note before the call that he believes Dawn of Titans will contribute about $25 million in its first year.

Gibeau said that CSR Racing 2 is going great in an interview earlier today, while he said the NaturalMotion team is working on boosting Dawn of Titans with live operations.