Lipofectin was kindly provided by Syntex, Inc., USA as an aqueous solution containing 1 mg I ml of 1 ipid ( DOTMA DOPE; 50 50 ). The procedure used was as described by Feigner et al., 1987 with appropriate I modifications as suggested in the user s notes. Lipofection was done with 0.5 x 106 cells seeded on a 60 mm plate. For each plasmid, the lipofection was performed in duplicate. The amount and quality of the plasmid DNA used ranged from 400 ng of crude DNA prepared by the mini prep method, to 5 ug of highly purified, cesium banded DNA. The appropriate amount of DNA was suspended in 1.5 ml of serum free DMEM. In another tube, 30 ug of lipofectin was suspended in 1.5 ml of serum free DMEM. The two solutions were mixed. The cells were washed twice with HBSS to totally wash off all traces of serum. The DNA 1 lipofectin mix was then applied to the cells and the cells incubated for 4 hours at 37°C. Next, 3 ml of media containing 10 % FCS was added and the incubation continued at 3 7°C for 16 hours. The culture supernate was then aspirated off and fresh medium added to the cells. The selection for stable clones was started after 48 hours by the procedure described below.