1. Seed approximately 106 cells on 100 mm culture plates in Dulbecco's modified Eagle medium (DMEM) containing 5% fetal calf serum and penicillin/streptomycin, and allow them to grow for twenty-four hours.
2. Transfect the culture with 10-20 μg of the plasmid-EcoGPT DNA.
3. After 3 days at 37°C in DMEM containing 5% fetal calf serum, trypsinize the cell monolayers and disperse 5 x 105 cells on fresh 100 mm culture plates in DMEM containing 10% dialyzed fetal calf serum supplemented with the MPA and Aminopterin solutions. This will give a final solution of approximately: 250 μg/mL xanthine, 15 μg/mL thymidine, 2 μg/mL aminopterin, and 25 μg/mL mycophenolic acid. Dialyzed serum is used to speed the selection process. Selection will be slower with undialyzed serum.
4. Twenty-four hours later, replace the medium with fresh medium containing the same supplements, and fluid change.
5. Repeat every 3 days. Colonies are visible in 7-10 days, typically.
Note: Other media bases can also be used such as Ham's F12. Much depends upon the cell type being used.