Over time, several supporting characters have been introduced and promoted to starring roles, including physicist Leslie Winkle, neuroscientist Amy Farrah Fowler, microbiologist Bernadette Maryann Rostenkowski-Wolowitz, comic book store proprietor and friend of the other characters Stuart Bloom. The series also features numerous supporting characters, each of whom plays a prominent role in a story arc. Included among them are parents of the main characters, their dates and their coworkers. Celebrities such as Stephen Hawking appear in cameo roles as themselves.