NANOG and infection: We began to add 2i/3i on day 4 after infection because previous reports demonstrated that KSOM-transduced MEFs underwent a mesenchymal-to-epithelial transition around day 5 after infection in the initiation phase, followed by the expression of SSEA1 and NANOG in the maturation phase (Li et al., 2010, Polo et al., 2010).