1. Transfer the media containing retroviral particles from plates to a sterile vessel and centrifuge the medium at 300 x g for 10 min to remove cell debris.

2. Filter the supernatant through 0.45µm filter.

3. Transfer filtered supernatant to a sterile vessel and add 1 volume of cold Retrovirus Precipitation Solution (4°C) to every 4 volumes of retrovirus-containing supernatant. (Example: 5ml Retrovirus Precipitation Solution with 20ml viral supernatant).

4. Mix well and refrigerate 3hrs to overnight. Retrovirus-containing supernatant mixed with Retrovirus Precipitation Solution are stable for up to 4 days at 4°C..

5. Centrifuge mixture at 1500 × g for 30 minutes at 4°C. After centrifugation, the retroviral particles may appear as a beige or white pellet at the bottom of the vessel.

6. Discard supernatant. Spin down residual solution by centrifugation at 1500 × g for 5 minutes. Remove all traces of fluid by aspiration, taking great care not to disturb the precipitated retroviral particles in pellet.

7. Resuspend retroviral pellets in 1/10 to 1/100 of original volume using cold, sterile PBS or DMEM at 4°C.

8. Aliquot in cryogenic vials and store at -80°C until ready for use.