PBS and then replenished with the complete medium. Two days following transfection, the cells were subcultured into the appropriate selective medium for selection of stable clones as described below.

Calcium phosphate mediated stable transfections were performed by the method of Graham and Van der Eb ( 1973 with modifications as described by Gorman ( 1986 ). For each plasmid, two petri dishes each containing 0. 5 x 106 CHO-K1 cells were used, with 10 ug of cesium purified DNA for each transfection. A mock transfection which did not contain any DNA, was performed simultaneously as negative control. Precipitation of the DNA was done with great care to ensure the obtention of a fine, translucent precipitate rather than a dense and opaque precipitate. The calcium phosphate I DNA precipitate was added in 4 ml medium to the cells and the cells incubated for 3 hours at 37°C. At this stage, the cells were examined under the microscope and a fine precipitate appeared as small grains all over the cells. The cells were washed once with serum free medium and a glycerol shock given for 3 minutes at 37°C. The cells were washed twice again with

rinsed twice with serum free medium and replenished with 4 ml of DMEM containing 10 % FCS and 100 uM chloroquine. The incubation was continued for another 3 hours at the cells were washed and fed with the normal growth medium containing 10 % FCS. As in the case of FWIL cells, the supernate was collected after 72 hours of transfection and assayed for BhCG activity by RIA.

ayed for BhCG activity by RIA. In case of the other five monolayer forming cell lines, a slightly different protocol was used. Only 1.8 ug plasmid DNA was used for each transfection using 0.5 x 106 cells, and 70 uM chloroquine was included in the DNA 1 DEAE-dextran mixture. Cells were fed 3 hours prior to transfection and washed twice with serum free medium just before exposure to DNA. Cells were exposed to DNA 1 DEAE-dextran mix for approximately 3 hours at 37°C. Following this, the cells were

1 - 5 ug of plasmid DNA using the DEAE -dextran procedure. DEAE dextran M.Wt. 500,000 was used to perform transient transfection by the method of Luthman and Magnusson 1983 ) , with modifications as described by Gorman ( 1986 ) . Six cell lines ( described above ) with two petri dishes ( 60 mm ) for each cell line were used. In case of FWIL, 5. 4 ug plasmid DNA was used to transfect approximately 5 x 106 cells. No exposure to chloroquine was given. The cells were treated with the DNA 1 DEAE -dextran mixture for 20 minutes at 37°C in a tightly capped tube, mixed gently and reincubated at 37°C for 10 minutes. The sample was then diluted with 3 ml of IMDM supplemented with 10 % FCS, centrifuged and the pellet washed once with normal growth medium. Finally, the pellet was resuspended in 4 ml of growth medium and transferred to a T-25 flask. After incubating for 24 hours at 37°C, 3 ml of fresh medium was added to the cells. The cells were harvested after 72 hours post transfection and the culture supernate was ass

Transient expression of the cloned gene product was studied by transfection performed with