The present invention provides a vector for expressing at least a first and second nucleic acid molecules which exhibit a percentage of homology of approximately 80% or greater than 80% over a portion of 40 or more continuous nucleotides and wherein said first nucleic acid molecule and/or said second nucleic acid molecule is modified so as to reduce said percentage of homology to less than 75%. The present invention also relates to substantially isolated nucleic acid molecules comprising a nucleotide sequence as defined in any of SEQ ID NO: 9-15 and 66-69. It also provides a host cell and a pharmaceutical composition comprising such a nucleic acid molecule or vector as well as their use for therapeutic or preventive purposes.
