The absorbance of positive DNA-protein-interaction is affected by blocking reagents. Changes in binding affinity and fold differences between positive and negative DNA-protein interaction for different blocking reagents are analysed with At BPC2 and GA- or GAm-probes (A. + B.): α-DIG blocking reagent (DIG-block), α-His blocking reagent (His-block), 1% bovine serum albumin (BSA) in TBS-T and 5% non-fat dried milk in TBS-T (milk). A. The average OPD-turnover is measured in an experiment over 60 min. B. The normalised values and standard deviations at 50 minutes incubation time are graphed as histograms for the four different blocking reagents. The background-normalised fold-differences are given above the respective columns. Representative wells of the microtiter plate are shown below the graph for visual inspection. The grey background indicates the negative control (untransformed BL21/RIL cells) reference values.