How did the stoic, time-traveling mutant soldier Cable and the wise-cracking, immortal mercenary Deadpool become friends when one was originally hired to kill the other? By surviving a horrible genetic accident that bound them together on a genetic level! Following the cancellation of both of their respective series, Cable & Deadpool was launched in 2004 where the former enemies met on the opposite ends of a dash for a dangerous new virus which threatened the safety of the entire world. Little did they know surviving this mission would plant the seeds for one of the most hilarious, unlikely, yet sincere and moving friendships in Marvel Comics history.
In "If Looks Could Kill" by Deadpool co-creator Fabian Nicieza and artist Patrick Zircher, Wade Wilson has been contracted by the One World Church to infiltrate a company called Sunic and steal the Facade Virus, a potent new disease that'll coincide with the Church's vision of achieving global equality by turning everyone blue. Unknown to Deadpool, his old enemy Nathan Summers aka Cable is also aware of the virus, having telepathically infiltrated the company with the goal of destroying it. Deadpool eventually gets the upper hand on Cable and gives it to the Church, who double-cross Wade, infecting both he and Cable with the virus to test it out.
As Deadpool and Cable are literally melting away in the ruins of the One World Church headquarters, Cable realizes their powers can actually save each other, gambling on a risky proposal that Wade can't help joke about. Using his powers, Cable absorbs Deadpool into him, their genetic structure temporarily bonding before Nathan vomits up Wade's essence, which heals back into his old body. As Cable gains better control of the Facade Virus and uses his powers to resurrect his old ship Graymalkin, he uses its teleportation matrix to go elsewhere. However, when Nathan teleports, it turns out Deadpool does too. They appear in the same location, graphically bonded together before tearing themselves free. The anti-heroes learn that due to being bonded genetically, Cable's machines recognize them as the same entity, so whenever either of them teleports, the other is along for the ride.
Deadpool and Cable shouldn't be friends, heck even in the beginning they hated each other. But Nicieza cleverly used this genetic accident to give them a legitimate reason to keep working together despite any of the obvious obstacles posed by their very different outlooks. Deadpool's advanced healing factor meant that he'd survive any gross side effects of the teleportation and always have some quip ready to go, while Cable had one of the Marvel Universe's deadliest operators by his side at a moment's notice. As Cable attempted to create a utopian society called Providence, Wade was there offering input and suggestions, whether Nathan asked him to or not. Any time either one of them teleported, the other followed, creating some hilarious hijinks that slowly created an unbreakable bond between the two.
Though complete opposites, Cable and Deadpool were forced to spend time with each other, hear each other out, and even come to rely on each other over time. It was this very dynamic that would lead to them becoming close friends - a relationship the Ryan Reynolds/Josh Brolin pairing of Deadpool 2 haven't quite managed to create yet. In the insane and unpredictable fashion that defines Deadpool, Cable ended up being the best friend he could have hoped for, helping him take responsibility for his life, and begin the move from dangerous antihero to well-meaning hero.