Recombinant vectors

A recombinant replication competent retrovirus comprising:

• a retroviral GAG protein;
• a retroviral POL protein;
• a retroviral envelope;
• a retroviral polynucleotide sequence comprising a Long-Terminal Repeat (LTR) sequence at the 3′ end of the retroviral polynucleotide sequence, wherein the 3′ LTR comprises a sequence that is at least 98% identical to the sequence from about nucleotide 9405 to about 9998 of SEQ ID NO:19 or 22, a promoter sequence at the 5′ end of the retroviral polynucleotide sequence, said promoter being suitable for expression in a mammalian cell, a gag nucleic acid domain, a pol nucleic acid domain and an env nucleic acid domain, wherein the promoter, gag, pol and env domains comprise a sequence that is at least 98% identical to the sequence from about 1 to about 8323 of SEQ ID NO:19 or 22 and wherein the retroviral polynucleotide sequence lacks 70 base pairs of MLV sequence downstream from the 3′LTR compared to a pACE vector of SEQ ID NO:21;
• a cassette comprising an internal ribosome entry site (IRES) comprising a sequence that is at least 98% identical to the sequence from about 8327 to 8876 of SEQ ID NO:19 or 22, operably linked to a heterologous polynucleotide, wherein the cassette is positioned 5′ to the 3′ LTR and 3′ to the env nucleic acid domain encoding the retroviral envelope and wherein SEQ ID NO:
• 19 and 22 lack small repeats on either side of the cassette compared to the pACE vector of SEQ ID NO:21;
• and
• cis-acting sequences necessary for reverse transcription, packaging and integration in a target cell,
• wherein the RCR maintains higher replication competency after 6 passages compared to a pACE vector comprising SEQ ID NO:21.