Relative expression level of traditional and novel reference genes during development. A, Relative expression levels for five traditional genes, i.e. ACT2 (At3g18780, black), TUB6 (At5g12250, red), EF-1α (At5g60390, green), UBQ10 (At4g05320, cyan), and GAPDH (At1g13440, blue). B, Expression levels for five novel genes, i.e. At4g34270 (black), At1g13320 (red), At1g59830 (green), At4g33380 (cyan), and At2g28390 (blue). The RMA-normalized ATH1 expression level for each gene in every sample (1–101) is expressed relative to the average expression level over the entire developmental series. The descriptions of the developmental samples 1 to 101 are available at http://www.weigelworld.org/resources/microarray/AtGenExpress/. The Affymetrix probe set for At5g60390 is not specific but detects transcripts of four EF-1α genes (see Table I).