PM- and MM-characteristics of washing: Mean probe intensity (panel a), the asymptotic washing level (panel b) and the initial washing decay time (panel c) are shown as a function of the set-averaged probe intensity, Σ, which roughly estimates the expression level of the respective probe set. All values are separately calculated for PM and MM probes. Their characteristics are essentially identical upon non-specific hybridization at small Σ-values. Beyond a threshold the data split into two branches due to the onset of specific hybridization. Washing removes specific transcripts more strongly from the MM owing to their weaker binding caused by their central mismatch.