Shares of Atara Biotherapeutics Inc. fell nearly 20 percent after the company announced it is looking for a new leader as its current chief executive prepares to step down.

The company, which is technically headquartered in South San Francisco but has large facilities in Westlake Village and Thousand Oaks, develops T-cell immunotherapies for cancer, autoimmune and viral diseases. Dr. Isaac Ciechanover was a founder of the company has served as chief executive for six years.

Ciechanover will continue as chief executive until the board names a successor or June 30, whichever comes first. The board has started the search for a new leader, the company said Thursday.

“It has been a privilege to lead Atara from its founding and through its transformation into a leading late-stage development company with world-class manufacturing capabilities,” Ciechanover said in a statement. “I look forward to working with the board to transition a successor who can build on all our success and guide Atara in the years ahead.”

Shares of Atara (ATRA) closed Thursday down $6.86, or 19.5 percent, at $28.40 on the Nasdaq.