Portrayed by actor William Sanderson, J. F. Sebastian is a gifted and painfully shy "genetic designer" in Ridley Scott's Blade Runner.

Described to Sanderson by Scott as "a totally innocent man," The character is an interesting one. Afflicted with Methuselah syndrome—a premature aging disease, Sebastian is Twenty-five, looks older, and acts like a genius on the border of savantism. He is employed by the Tyrell Corporation, working under Eldon Tyrell himself as one of his top genetic designers. Sanderson has stated that he based his portrayal of the character on two things: A physical image of Albert Einstein, and a video of a child suffering from progeria, a disease that prematurely ages children.

J.F. lives alone in the famous Bradbury building in L.A. His huge apartment is populated by strikingly real (and really creepy) animal/human toys he designs and makes as friends.