The U.S. Food and Drug Administration said on Tuesday it had named Scott Steele as acting director of its Center for Biologics Evaluation and Research (CBER), following the exit of top vaccine scientist Peter Marks.

Dr. Peter Marks, the top vaccine official at the Food and Drug Administration, wrote in a letter that he was resigning from his position.

Dr. Peter Marks, the Food and Drug Administration’s top vaccine regulator, has resigned, an official at the Department of Health and Human Services said Friday.