Figure 5.

Problems with the two-tailed Mann-Whitney test. Motif 1 and Motif 3 would have the same p-values with the two-tailed Mann-Whitney test since only the t-value 0.9 is important and all other t-values from Motif 1 or Motif 3 are symmetric with respect to 0 contributing the same to the rank sum as does t-value 0 even though genes in Motif 1 show higher magnitude changes than genes in Motif 3.