At this point in its run, the X-Men franchise must be imbibing a little of Wolverine's healing factor.

These movies have been a fixture since the original "X-Men" exploded onto the scene in 2000. They've weathered every misstep and survived the rise of Marvel Studios and the Avengers. And now "X-Men: Apocalypse" is here to introduce a whole new wave of mutant heroes and the biggest, baddest X-Men villain of them all.

To celebrate this big release, we're looking back at the five core "X-Men" movies released so far and explore some of the interesting trivia and hidden cameos you might not know about.

'X-Men' (2000)
1. Hugh Jackman wasn't the first choice to play Wolverine. Actor Dougray Scott was originally cast in the role, but he dropped out because of a conflict with the filming of "Mission: Impossible II." We imagine he regretted that decision later.

2. Joss Whedon penned a rewrite of the screenplay. Whedon's draft was rejected by the studio because of its "quick-witted" approach to the source material. Somehow, Marvel Studios didn't seem to mind that approach when Whedon directed "The Avengers" 12 years later.

3. Several major characters (Beast, Nightcrawler and Pyro) and other iconic X-Men elements (including the Danger Room) were cut from the script before filming because of budget concerns.

'X2: X-Men United' (2003)
4. This sequel is one of the major reasons leading to Marvel's decision to reveal Wolverine's origin story in the comic book, "Origin." Many within the publisher feared that the movies would end up revealing this murky period of Wolverine's past before they got the chance.

5. Rather than resort to digital trickery, director Bryan Singer relied on a group of trained mimes to play the museum crowd that Xavier freezes in place with his psychic powers.

6. In the scene where Mystique accesses Yuriko's computer terminal, viewers are treated to a number of references to other Marvel characters. Among many other things, the computer files reference Gambit, Mister Sinister, Omega Red, Project Wideawake, the Von Strucker twins, and even Franklin Richards from the "Fantastic Four" franchise.

'X3' (2006)
7. Alan Cumming was expected to reprise his role as Nightcrawler in this sequel. However, because the role was so small and Cumming disliked the painstaking process of applying Nightcrawler's makeup, he opted not to return. The prequel video game "X-Men: The Official Game" offered a story-based explanation for why Nightcrawler was absent.

8. When Singer dropped out of this sequel to direct "Superman Returns," Fox approached a number of potential replacements before ultimately choosing Brett Ratner. That list included Peter Berg, Joss Whedon, Alex Proyas, and 'X-Men: First Class" director Matthew Vaughn.

9. Gambit was originally intended to appear in this sequel as one of the X-Men and a rival for Rogue's affections. Though the role was written out of the script, Gambit does have a small cameo in the novelization.

'X-Men: First Class' (2011)
10. This prequel started life as a Magneto-centric story in the vein of "X-Men Origins: Wolverine." But in light of the lukewarm reception to that movie, the Magneto project evolved into an ensemble X-Men movie.

11. While there was once a Marvel comic called "X-Men: First Class," it featured an almost completely different cast of mutant heroes, including Cyclops, Jean Grey, Iceman, Angel, and Beast.

12. The film's original ending featured a "psychic" battle between Emma Frost and Charles Xavier, involving a city of the mind that the two would battle each other within -- think punching each other into buildings or with buildings. The filmmakers were very excited to film, but never got the chance. Why? Fox cut it when "Inception" came out because of similar sequences in that film. Womp womp.

'X-Men: Days of Future Past' (2014)
13. Wolverine's apartment in 1973 is decorated with many references and homages to his Japanese travels, including samurai swords and a photo of Mt. Fuji. Even the red and yellow color scheme pays tribute to his 1980's-era costume from the comics (a costume that he has yet to actually wear in the films).

14. When Mystique infiltrates the army base in Vietnam, her cover identity is "Col. Sanders." Apparently she loves those 11 herbs and spices.

15. Quicksilver's appearance in both this film and "Avengers: Age of Ultron" created some tension between Fox and Marvel Studios. The two studios hammered out an agreement that stated that the X-Men movies can't reference Quicksilver's status as an Avenger, while the Avengers movies can't acknowledge his mutant heritage or the fact that Magneto is his father.