Spike-in array. This scatterplot shows the design of the spike-in experiment. The log fold change for each probe (M) is shown as a function of the mean single channel intensity (A). This array was pre-processed using loess normalization without background subtraction. R and G indicate spike-in concentration levels in the red and green channels. Observed fold changes reflect spike-in concentrations with the exception of the blue points in the lower left and to some extent the orange points in the same area. This array is representative of the spike-in arrays in this experiment.