Kruppel like factors (Klfs) are dynamically expressed during the first few days of ES cell differentiation. (A) GeneSpring plot of normalized gene expression for all KLFs detected during ES cell differentiation in serum or serum-freeB4L culture. Most of the genes are listed on the y-axis in order from their highest relative expression in ES cells. There is dramatic up regulation of Klf1 (Eklf) only in serum following day 6 of differentiation. Plots representing Nanog (red), Sox2 (red) and Pou5f1(Oct4) (green) are shown for comparison. Pou5f1 and Nanog gene expression persists at high levels for 2–3 days after Klf2, Klf4 and Klf5 are down regulated. (B) Validation of changes in gene expression of six members of the KLF family by quantitative real time RT-PCR. Scheme as described for Figure 2B.