Maximum a posteriori Beta pdfs of classes in main prototype cases. The predictive pdf and the Bayes Error Rate E of both tumoral (T) and normal (N) classes, for examples tags contained in the three important prototype cases described in text, are shown in the figure. The 'x' and 'o' marks represent observed abundances in each tumoral and normal. Frame a) shows case (i) example when methods agree with "no differential expression" conclusion. Frame b) shows case (ii) example when methods agree with "differential expression" conclusion. Frame c) and d) show case (iii) examples when classical P-value method leads to significant differential expression between classes and our method indicates pdf superposition if one take within-class variability into account. Individual observations indicate that the classes are not clearly divided, casting doubt on "differential expression" conclusion.