Expression stability of housekeeping genes. Results of QPCR analyses from different (A) stages of development (Figure 1); (B) treatment conditions (Table 3); and (C) tissue types (Table 2, male-female data combined) were used to calculate stability using geNorm. For details see Methods and Results. The relative M-values (y-axis) are defined as a measure of gene expression stability, with an increasing M-value correlating with less stability. Note that the range of M-values (high to low), indicating the degree of variability between the least and most stable genes, differed for each of the three conditions: development (3.4-fold), treatment (3-fold), and tissue distribution (4.5-fold).