Klf6^-/- ES cells proliferate more slowly than Klf6^+/+ ES cells. ES cells were plated on either MEF feeder cells (A) or gelatinized dishes (B) at the concentration of 1 × 10⁵ cells/well of 6-well plates. Undifferentiated ES cells were counted after 72 hours of culture, and doubling times were calculated. Averages of 9 wells from 3 independent experiments are indicated. Error bars represent SE. ^*P < .05; ^**P < .01 (Student t test). (C) To calculate the growth curve of EBs, 2 × 10⁴ cells were cultured in differentiation medium. EBs were harvested and cell numbers were counted at each time point. Each symbol represents the average of 3 plates.