Selectivity distribution for the Agilent probe set. The black line illustrated the selectivities (Eq. eq:Selectivity) of all Agilent probes when the array is hybridized to yeast cDNAs at the initial concentrations described in Methods. The gray lines are results for four different initial initial conditions. The red line delimits 90% selectivity. The distribution is independent of the initial concentrations (x-axes are different for all five sets of results). About 100 probes will not be selective in any given microarray experiment.