Assay sensitivity. Real-time RT-PCR after titration of cancer cells (MDA-MB-435) into blood from a healthy donor. Each data point is the average of three experiments. The PCR reaction was run over 40 cycles, and the reverse transcription reaction contained the standard amount of 1 μg RNA in a 20 μl reaction. The insert shows the data replotted on a double-logarithmic scale. The x-axis labels are 10×.