Stan Lee, the famed and creative genius behind Marvel Comics, died on Monday at 95 years old.

Lee was taken by ambulance to Cedar-Sinai Medical Center in Los Angeles where he later died. The cause of death is not yet known, according to Kirk Schneck, an attorney for Lee’s daughter.

Lee first began his career in 1939 at Timely Comics where he was a writer, editor and occasional illustrator. At the time, National Comics (later DC Comics), dominated the superhero industry. Lee was propositioned with designing a competing set of superheroes against the DC Justice League.

The theory behind his superheroes which made them stand out in competition with DC was they were more relatable; often embodying personal flaws and characteristics. Often the characters lived in New York City, as opposed to the Justice League, which dwelled in Gotham or Metropolis.

Between 1961 and 1964, Lee introduced the Fantastic Four, Spider-Man, the Hulk, Iron Man, Thor, the X-Men and Daredevil. The success of his characters dominated the comic industry and surpassed DC Comics in popularity.

Of all the superheroes, it was Spiderman who was the most notable character. His alter-ego was Peter Parker, a photographer who gained his crime-fighting characteristics after being bitten by a radioactive spider. His call-of-duty didn’t necessarily resonate in crime-fighting; he was always concerned about his Aunt, and had a boss that breathed heavily down his neck.

Lee said in an interview with CNN in 2006, that of all the characters he created, Spiderman was the least likely he’d thought would be such a success. “I never thought that Spider-Man would become the worldwide icon that he is. I just hoped the books would sell and I’d keep my job,” he said.

He became the “face” of Marvel Comics after reigning in the success of his superhero writing, editing and creation. Beginning in 1972, he became the editorial director and publisher. The company’s branding and unification of fans came in the form of the exclamation of “Excelsior!”

It could be heard at comic book conventions and events around the world.

Last year, Lee lost his wife Joan, to whom he had been married to for 69 years. She died at age 95 as well. Marvel fans came to know Joan the brand just as much as they knew Lee the brand. She made a cameo in “X-Men: Apocalypse” in 2016 and did voice-over work in animated versions of “Spider-Man” and “Fantastic Four.”

Both Lee and Marvel enjoyed the success of comic books the form of multi-billion dollar blockbuster movies. Walt Disney Company purchased Marvel Entertainment in 2009 for $4 billion.

In a YouTube video from 2012, Lee fervently exclaimed to his followers that the medium in which he gained success should be spelled as one word:

“They are not funny books,” he said. “They are not comic books, they are comicbooks! Remember that, or incur my wrath.”