Mutants (1974) by Robert Silverberg

Finally, after years of struggle, mutants are being accepted into the mainstream of human society. But just as things seem to be settling down rumors of strange happenings in the New Mexican desert start raising questions about mutants once again.
Rick Akimura, the long-sought supermutant, has established a cult-like base in the desert and once again threatens the fragile peace between mutants and normal humans. But Joachim Metzger, leader of the mutants, is determined to maintain the hard earned peace with the help of Rick's own brother, the brilliant therapist Julian Akimura.