Indeed, following our line of argument, Figure 2 has even more relevance when we highlight the fact that the 5,153 probes have not been selected with a supervised method. The authors say that the only selection criteria was to single out the 5,153 cDNAs whose expression varied most across samples. In the supplementary notes of their paper the authors say: ''We included for subsequent analysis only well measured genes whose expression varied, as determined by (1) signal intensity over background .1.5-fold in both test and reference channels in at least 75% of samples, and (2) 3-fold ratio variation from the mean in at least two samples; 5,153 genes met these criteria.'' As a consequence, Figure 2 has been generated without class selection bias only using the genes that have the most varied expression pattern.