Deadpool's skin was altered due to exposure to a mutagenic agent called Weapon X. This experimental program, conducted by the Canadian government, was designed to enhance human abilities and create super-soldiers.
The process involved injecting the subject with a super-soldier serum and then exposing them to a mutagenic agent. This agent, in Deadpool's case, caused his skin to become regenerative, giving him the ability to heal from almost any injury. However, it also left him with disfigurement and a unique appearance.
The Weapon X program is a significant element in Deadpool's origin story, shaping his physical abilities and his personality. The experiment's consequences gave him a unique power, but they also left him with scars, both physical and emotional.
