Crabp1 negatively modulated stem cell proliferation. The top panel depicts the ESC-neuronal differentiation procedure. (a) Crabp1 messenger RNA (mRNA) expression pattern in WT ESC differentiation into immature neurons. (b) Protein levels of P-ERK1/2, ERK1/2, Pax6, Crabp1, and GAPDH in ESCs and neurospheres. (c) NSC marker nestin mRNA expression pattern in ESCs and neurospheres. (d) Expression patterns of proliferation markers Ki-67 (ES cells, P = 0.016; EBs, P < 0.001; neurospheres, P = 0.002) and PCNA (ES cells, P < 0.001; EBs, P < 0.001; neurospheres, P = 0.016). Results are presented as mean ± standard error of the mean. *P < 0.05, **P < 0.01 compared with the WT group. GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PCNA, proliferating cell nuclear antigen; P-ERK1/2, phospho-extracellular regulated kinase 1/2.