Retroviruses are ubiquitous viruses, found in fish, avian and mammals. The diameter of the retroviral virion is approximately 80 – 100 nm. The outmost layer of the virus is a lipid bilayer derived from the host cell membrane. It surrounds the spherical capsid in which two identical single stranded RNA strands are located.

They are positive-sense and approximately 7 – 11 kB long (5)

Generally the retroviral life cycle can be divided in two distinct phases: infection and replication. In the following sections we will focus on the Molony Murine Leukemia Virus, which is the origin of the viral vector we are actually using.

Viruses are classified as ecotropic, polytropic or amphotropic depending on the number of different host cells they are able to infect (ref). The Molony Murine Leukemia Virus (MMLV) is an ecotropic gammaretrovirus; it only infects rodent cells (mice and rat)(ref). This tropism of the virus is granted by its high specificity towards the mCAT1 receptor (ref). Once attached to the receptor the MMLV outer membrane fuses with the host cells membrane and the single stranded viral RNA genome is transferred into the cytoplasm. Here it is reverse transcribed into ds DNA by the viral reverse transcriptase.

MMLV is a simple retrovirus naturally encoding only three genes called gag, pol and env. Besides this trans-gene elements there are also regulatory cis-gene elements. The trans-gene elements are flanked by long terminal repeats (LTR). The 5’ LTR harbors a promoter region, which initiates transcription of the provirus, whereas the 3’ LTR is needed for polyadenylation of the proviral mRNA. The gag (group antigenes) gene is precursor polyprotein which is further processed after translation. After procession it forms the major proteins building up the core particle, RNA binding proteins and the nucleoprotein core particle.

Retroviral vectors are used for the delivery of variable gene cargo into cells, a process termed transduction. To ensure safe handling the cis- and trans-acting gene elements are separated from each other. Helper viruses or packaging cell lines, carrying the trans-acting gene-elements are needed for producing the functional viral vector. Besides safety issues this approach also has the advantage that, due to the deletion, a larger gene cargo (~ 8 kB) can be integrated into the vector (4). Our MLV vector needs the Phoenix cell line in order to replicate.