Novel proteins that constitute modified forms of a Neisseria meningitidis surface antigen and encoding nucleic acids are provided. The modified surface proteins are characterized by having deletions of non-conserved amino acids, and thereby being capable of eliciting cross-protective immune responses against Neisseria meningitidis. The invention extends to the use of the modified surface antigens in diagnostics, in therapeutic and prophylactic vaccines and in the design and/or screening of medicaments. The modified surface antigens are particularly useful in vaccines which effectively immunize against a broader spectrum of N. meningitidis strains than would be expected from a corresponding wild-type surface antigen.