Table 1. Characteristics of transformed CHO cells.

G-418 was used at a concentration of 1000 μg/ml for selection and 800 μg/ml for maintenance. Zeocin was used at a concentration of 600 μg/ml for selection and 400 μg/ml for maintenance. Hygromycin B was used at a concentration of 1000 μg/ml for selection and 800 μg/ml for maintenance. The same number of cells for each transformed line was plated out into multiwell plates and cultured for 3 days in medium containing the appropriate antibiotics at their maintenance concentrations (see Experimental section for details). The cells were then harvested and the viable (Trypan Blue excluding) cells counted. The values are expressed as means±S.D. (n=6) and are percentages of control value (i.e. CHO-O cells cultured without additional antibiotics), which was normalized to 100%. The probabilities of significance of the differences between the means of the transformed lines and the control were determined by the Student-Newman-Keul's multiple comparison t test. NS, not significant.

Cell line	Enzymes expressed by stable transformants	Antibiotics used for selection and maintenance (μg/ml)	Effect on cell division (%)
CHO-O	None (non-transformed cells)	None	100±13.7
CHO-A	AGT	G-418 (1000/800)	107.8±5.4 (NS)
CHO-G	GO	Zeocin (600/400)	105.1±5.3 (NS)
CHO-H	GR/HPR	Hygromycin B (1000/800)	67.4±4.7 (P<0.001)
CHO-AG	AGT and GO	G-418 and zeocin	64.3±2.4 (P<0.001)
CHO-AH	AGT and GR/HPR	G-418 and hygromycin B	76.9±1.9 (P<0.005)
CHO-GH	GO and GR/HPR	Zeocin and hygromycin B	75.2±8.9 (P<0.005)
CHO-AGH	AGT, GO and GR/HPR	G-418, zeocin and hygromycin B	48.5±5.6 (P<0.001)