Figure 1.

The reprogramming assay has revealed enhancer and replacement factors. (a) (i) Example characterization of enhancer factors (X and Y). Factors delivered using individual retroviruses expressing the relevant genes. Nanog serves as a marker of fully reprogrammed cells. Enhancer factors may act through proliferation-dependent (X) or proliferation-independent mechanisms (Y), both of which would increase the proportion of induced pluripotent stem cell colonies. (ii) Example growth curves for mouse embryonic fibroblasts infected with vectors expressing Oct4, Sox2 and Klf4 (O, S and K), and X, Y or control, displaying how proliferation effects can be measured. Error bars represent standard deviation. (b) Example characterization of a Sox2 replacement factor (Z). Error bars represent standard deviation.