Hybridization signal versus duplex stability. The sigmoid transfer function θ() of the Langmuir isotherm (right scale) has a narrow transition region ( ≈ 6 kcal/mol at a temperature of 310 K and a target concentration of 1 nM). Microarray hybridization signals (left scale) for incrementally increased duplex stabilities: The probe sequence motif was translated along the target sequence in increments of two bases (see inset), thus providing a set of different curves. All probes were hybridized with the common target sequence URA (1 nM in 5 × SSPE, for 20 minutes at 45°C). The approximately linear increase of the hybridization signal in the transition region extends over at least 13 base pairs ( ≈ 20 kcal/mol).