(a) Competitive PCR analysis of β2-microglobulin mRNA levels in MDA-MB-231 cells treated with 6-thioguanosine. Ten nanograms of cDNA from MDA-MB-231 cells treated with 10 μM 6-thioguanosine were amplified with five fourfold serial dilutions of pQA1 internal standard (IS). The cDNA was amplified under the following PCR conditions: 32 cycles, annealing temperature of 56°C, and 2 mM MgCl₂. (b) Effect of purine, 6-thioguanine, and 6-thioguanosine on steady-state mRNA levels of PTHrP in MDA-MB-231 cells. After normalization of cDNA to β2-microglobulin from MDA-MB-231 cells treated with 0.1, 1, 10, or 100 μM purine, 6-thioguanine, or 6-thioguanosine, samples were amplified under the following semiquantitative PCR conditions: 32 cycles, annealing temperature of 56°C, and 2 mM MgCl₂. Human specific sense and antisense primers to PTHrP were used. The figure represents the detection and quantitation of steady-state mRNA levels of PTHrP when treated with 100, 10, 1, and 0.1 μM purine, 6-thioguanine, or 6-thioguanosine, expressed as treated-to-control (T/C) ratios. Purine did not affect steady-state mRNA levels of PTHrP, whereas 100, 10, and 1 μM 6-thioguanine and 6-thioguanosine reduced steady-state mRNA levels of PTHrP by as much as 50%.