TABLE 1.
Gene amplification in REF52 cells pretreated and selected with PALA or MTXa
| Pretreatment | Selection | No. of colonies observedb | No. of cells analyzedb | Amplification frequency |
|---|---|---|---|---|
| None | PALA (30 μM) | 0 | 5 × 107 | <2 × 10−8 |
| MTX (50 nM) | 0 | 5 × 106 | <2 × 10−7 | |
| MTX (40 nM) | 3c | 5 × 106 | <2 × 10−7 | |
| PALA (15 μM) | PALA (30 μM) | 82 | 7 × 105 | 2.9 × 10−4 |
| PALA (10 μM) | PALA (30 μM) | 98 | 6 × 105 | 4.1 × 10−4 |
| PALA (15 μM) | MTX (40 nM) | 0 | 1 × 106 | <10−6 |
| MTX (50 nM) | 0 | 1 × 106 | <10−6 | |
| MTX, 15 nM | MTX (40 nM) | 0 | 1 × 106 | <10−6 |
| MTX, 20 nM | MTX (50 nM) | 0 | 1 × 106 | <10−6 |
| MTX, 15 nM | PALA, 30 μM | 0 | 1 × 106 | <10−6 |
| MTX, 20 nM | PALA | 0 | 1 × 106 | <10−6 |
a
REF52 cells were seeded at a low density (5 × 103 cells per 10-cm plate) in low, nonselective concentrations of PALA or MTX for 72 h and then exposed to selective concentrations. Resistant colonies appeared after about 4 weeks.
b
Totals from two or three independent selections.
c
dhfr was not amplified in these colonies.