The present invention concerns the technical field of nucleic acids and expression-optimized nucleic acids. The present invention concerns especially nucleic acids comprising a mutated foamy viral envelope gene encoding a foamy viral envelope polypeptide, which comprises a leader peptide (LP), a surface unit (SU) and a transmembrane domain (TM). The present invention also relates to modified polypeptides encoded by these nucleic acids. Furthermore, the present invention regards a method for preparing pseudotyped vector particles as well as a method for treating a genetic disorder comprising administering a nucleic acid or a polypeptide encoded by that nucleic acid.