A real-time PCR reaction was also performed on these 48 samples. Twenty-four replicates were used for each sample. Although the average copy numbers were close to the digital array data, large fluctuations (SDs of up to 0.5) were observed in the 24 reactions of each sample. Studies on other genes (for example, CYP2D6) showed that real-time PCR does not always produce accurate results (data not shown).