He helped the X-Men again against Bastion and his Sentinels. As a consequence, he was infected by Sentinel programming, assuming a more robotic look in the subsequent series "X-51", and losing self-control whenever he was faced with a mutant. During this series he was on the run from Sebastian Shaw, who wants his technology for himself. Because of his new programming, while seeking aid from the Avengers, he attacks Justice and Firestar. Because of his actions against Justice and Firestar, X-51's membership in the Avengers is revoked. At the end of "X-51", X-51 encountered one of the monoliths and disappeared, brought into the presence of the monolith's creators, the cosmic beings known as the Celestials.