Boxplots of up-regulated genes after normalization. Each box plot represents the M values of the up-regulated genes after print-tip normalization as the percent of genes differentially expressed is varied. If the normalization procedure is robust to the outliers then M = 2. (a) In this scenario print-tip lowess is robust to approximately 20% of differentially expressed genes when the default number of iterations is equal to 3. (b) For 10 robustifying iterations the normalization is reliable for approximately 25% of genes differentially expressed.