Fig. 3. Erk1/2 activity in ES cells is FGF dependent. (A-D) Immunostaining (left; phase-contrast, right) for phospho-Erk1/2 (pErk1/2) in mouse ES cells cultured in serum plus LIF for 14 hours in vehicle control (DMSO, A,C), inhibitor of MEK activation PD184352 (B), or FGFR inhibitor SU5402 (D). Each pair of images was taken at the same exposure time. Scale bar: 100 µm for A,B; 50 µm for C,D. (E) Immunoblot of Fgf4^-/- and Fgf4^+/- ES cell lysates cultured in serum-free N2B27 medium with or without LIF for 24 hours.