New X-Men, Vol. 3: New Worlds

His name is Charles Xavier … and he is a mutant.
For years, Professor X was widely regarded as an authority on genetic mutation, and an advocate of peaceful relations between man and mutant. The general public was unaware he possessed superhuman powers until the psionically empowered Cassandra Nova — his twin sister — addresses the global media while in possession of her brother's body and broke the decades-long silence. But in attempting to expose Xavier to his enemies, she unwittingly freed him from his self-imposed exile. From the teeming city streets of New York's Mutant Town to the irradiated ruins of the island nation of Genosha, once a haven for mutants, the X-Men set off to heal the rift between humanity and Homo Superior — only to discover a terrifying new threat from a new species!

Collecting: New X-Men (2001) 127-133