Pairwise correlations of expression levels for 100 maintenance genes. Expression levels for 100 maintenance genes were determined for 34 airway epithelial samples of which 24 randomly selected samples passed the pre-determined QC criteria and 10 failed one or more of the criteria. The vertical and horizontal numbers refer to the 34 samples, categorized as "pass" or "fail"; LA = large airway; SA = small airway. Pearson correlation coefficients for all pairwise comparisons between the 34 samples were determined and are plotted in grey-scale, with each cell representing a single correlation between two samples (white, r > 0.94; gray, 0.92 ≤ r ≤ 0.94; black, r < 0.92). Shown are the 24 × 24 comparison of samples both passing QC, the 24 × 10 between samples passing QC and samples failing QC, and the 10 × 10 comparison of samples both failing QC. Note that all of the correlation values <0.92 are derived only from pairwise comparisons including samples failing the QC criteria.