This address is the fictional location of the X-Mansion (a.k.a Xavier Institute for Higher Learning, formerly Xavier's School for Gifted Youngsters).
Originally in the Xavier family for 10 generations, Prof. Charles Xavier opened it as a school/training facility for mutants. The X-Mansion has taken on several different names and roles throughout the years, but the tenants are usually mutants doing mutant stuff.
Created by writer Stan Lee and artist Jack Kirby, The X-Mansion first appeared with the X-Men in X-Men #1 (Sept. 1963).
