Importantly, in concordance with the assessment of a loss of GFP expression from the retroviral vectors, analysis of transgene expression in the control and Fragile X syndrome iPSC clones using RT-PCR and primers specific for transgene cMYC, OCT4/POU5F1, KLF4, SOX2 indicated a silencing of their expression (Fig. S2). The gene discussed is POU5F1; the disease is fragile X syndrome.