What are retroviruses and why are they used as vectors?

Retroviruses are single-stranded positive-sense RNA viruses with a DNA intermediate. When a retrovirus infects a cell, the RNA genome is reverse transcribed into DNA, which subsequently integrates within the host cell’s genome (this combination of viral and host genetics is termed the provirus).¹ Genomic transcription of viral genes then generates viral genomic RNA or RNA encoding viral proteins. This ability to permanently alter the host genome, as well as the simplicity with which retroviruses can be converted into replication-defective vectors, makes them well suited for viral vector work.¹ Retroviral genomic integration into the host genome does raise the possibility of mutagenesis and oncogenesis.¹ In addition, retroviruses are only able to transduce dividing cells, limiting the potential of retroviral vector-based gene therapy approaches in primary cells, neurons, and other slowly dividing cells.²

References:
1. D.S. Anson, “The use of retroviral vectors for gene therapy-what are the risks? A review of retroviral pathogenesis and its relevance to retroviral vector-mediated gene delivery,” Genet Vaccines Ther 2: 9, 2004.
2. J.M. Coffin, et al., “Retroviruses,” Principles of Retroviral Vector Design. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.