SOX2 and Down syndrome: Normal (Twin-N) and Down syndrome (Twin-DS) fetal fibroblasts were isolated from monozygotic twins discordant for trisomy 21 (Dahoun et al, 2008) and used to establish Twin-N-iPSCs and Twin-DS-iPSCs using OCT4, SOX2, KLF4 and c-MYC genes as previously described (Takahashi et al, 2007; Grad et al, 2011; Fig 1A and B).