From CNN:

“As part of the investigation into Chris Cuomo’s tenure at CNN, I was asked about a consensual relationship with my closest colleague, someone I have worked with for more than 20 years,” Zucker told employees in a memo. “I acknowledged the relationship evolved in recent years. I was required to disclose it when it began but I didn’t. I was wrong. As a result, I am resigning today.”

Zucker did not name his colleague, but the relationship is with Allison Gollust, his key lieutenant for the last two decades. Gollust is remaining at CNN.