EIAV vectors codon optimized.

Use of a nucleotide sequence coding for retroviral gag and pol proteins capable of assembly of a retroviral vector genome into a retroviral particle in a producer cell to generate a replication defective retrovirus in a target cell, wherein the nucleotide sequence is optimized in its codons for expression in the producer cell, and wherein the nucleotide sequence comprises the sequence shown in SEQ ID NO: 16.