POU5F1 and neoplasm: Since OCT4 was only phosphorylated at S229 in the G2/M phase of the cell cycle in mouse ESCs [34,35] and only a small portion of OCT4-positive tumor cells were positive for p-OCT4(S236) in a GCT sample (Figure 1A), we inferred that the phosphorylation of OCT4(S236) in human cancer cells would also occur at the G2/M phase.