Statistical analysis

All values represent means ± standard error of the mean. Significant differences were tested by unpaired Student t-test. A P value less than 0.05 was considered statistically significant. Linear regression statistics were done using an F-test, comparing the ratio of two variances (Supplementary Figure S1). We considered a theoretical slope equal to 1 if the values obtained for the cassette titration were equal to the values obtained for the ITR2 titration. The cassette/ITR2 slope calculated from experimental qPCR data was 0.2022 (<0.4236 with 95% confidence intervals). The P value calculated from the F-test and the degree of freedom was P < 0.0001.