Retroviral gene transfer vectors

A retroviral vector which contains a nucleotide sequence N of the general formula

• 5′-end—(5′-UTR)-(gag)-(ex)-(vir)—3′-end
• wherein
• the presence of (gag) is optional,
• (5′-UTR) is a non-coding nucleotide sequence which in retroviruses lies directly 5′ of the start codon of the gag gene,
• (gag) is, when present, a section of the nucleotide sequence which codes for the gag gene,
• (ex) is a non-viral nucleotide sequence and
• (vir) is a retroviral nucleotide sequence,
• wherein
• the number of nucleotides in (gag) is less than 400 bp, and the vector contains no ATG or AUG upstream of the translation start of the sequence in (ex).