Early Monday morning, at the age of 95, Stan Lee died at Cedars-Sinai Medical Center in Los Angeles, a representative for the Marvel mastermind's family confirmed to The Hollywood Reporter. The sad news arrives several months after the groundbreaking and prolific comic book writer and editor had been hospitalized in early February after experiencing an irregular heartbeat and shortness of breath. That time, he spent only a few hours at Cedars-Sinai before returning home and reporting that he was "feeling good."

According to The New York Times, Lee began dabbling in creating superhero stories in the 1940s, honing his skills until the release of the first issue of The Fantastic Four, with frequent collaborator Jack Kirby, in 1961. In the ensuing decades, Lee introduced many more now-iconic superheroes—including Black Panther, Thor, Iron Man, Spider-Man, the X-Men, and the Incredible Hulk—and worked to bring them to both big and small screens. Those Hollywood endeavors hit their stride only in the last decade, with the creation of the immensely successful Marvel Cinematic Universe franchise. Though he was almost always credited as an executive producer for the MCU films, Lee's best-known role was likely that of himself: Since 2008's Iron Man, he'd made fan-favorite cameos in every MCU movie, as well as in several other films and TV episodes set in the extended Marvel universe.

After news broke of Lee's passing on Monday, comic book fans paid tribute to the man behind their favorite characters. Among those offering condolences were many members of the Marvel universe, including Chris Evans (Captain America), Ryan Reynolds (Deadpool), Letitia Wright (Black Panther's Princess Shuri), Robert Downey Jr. (Iron Man), and Tom Holland (Spider-Man). "There will never be another Stan Lee. For decades he provided both young and old with adventure, escape, comfort, confidence, inspiration, strength, friendship and joy. He exuded love and kindness and will leave an indelible mark on so, so, so many lives," Evans tweeted, adding Lee's signature sign-off and motto: "Excelsior!!"

Many more stars not involved in the MCU also expressed their sadness over the loss of Lee, praising him for his imagination, for his creativity, and, as Seth Rogen put it, "for making people who feel different realize they are special."

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