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

Using calcium phosphate.