Expression of CREs with four unique binding sites. Expression of the 20 CREs with one binding site for each of the four TFs in the OSKE library. In CRE names: (O) POU5F1 (OCT4) binding sites; (S) SOX2 binding sites; (K) KLF4 binding sites; and (E) ESRRB binding sites. Lowercase letters indicate the reverse-orientation. Expression represented as mean ± SEM.