The Mutant Registration Act was a debated law proposed to Congress by Senator Robert Kelly, advocating for all mutants to be identified in an official census documenting their numbers, abilities and whereabouts circa 2003.
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Kelly's promotion of the Mutant Registration Act led to Magneto singling him out, and he ordered Toad and Sabretooth to capture him. After Senator Kelly died, Mystique impersonated him and called off the Registration Act.
Following the Liberty Island incident and the capture of Magneto, William Stryker's planned assassination of President McKenna by using Nightcrawler is theorized by Storm to potentially be responsible for a reintroduction of the Mutant Registration Act, or worse, the placement of every mutant under arrest. Stryker meets with President McKenna at the White House following the botched assassination, he is introduced to Senator Kelly (Mystique still posing as the deceased Senator), whereupon he comments that Kelly used to be a staunch supporter of the Registration Act, but his ideas on the mutant problem have since changed.