Primary human dermal fibroblasts were obtained from the Coriell Institute Cell Repository, the University Hospital in Erlangen, DNA and Cell Bank of ICM in Paris, and ATCC (Table S1). Protocols were previously approved by the Salk Institute Institutional Review Board and informed consent was obtained from all subjects. Cells were cultured in DMEM containing 15% tetracycline-free fetal bovine serum and 0.1% NEAA (Life Technologies), transduced with lentiviral particles for EtO (Ladewig et al., 2012; Mertens et al., 2013a) and XTP-Ngn2:2A:Ascl1 (N2A), and expanded in the presence of G418 (200 μg/ml; Life Technologies) and puromycin (1 μg/ml; Sigma Aldrich). For iN conversion, fibroblasts were pooled into high densities and after 24 hr the medium was changed to neuron conversion (NC) medium based on DMEM:F12/Neurobasal (1:1) for 3 weeks. NC contains the following supplements: N2 supplement, B27 supplement (both 13; GIBCO), doxycycline (2 μg/ml; Sigma Aldrich), Laminin (1 μg/ml; Life Technologies), dibutyryl cAMP (500 μg/ml; Sigma Aldrich), human recombinant Noggin (150 ng/ml; Preprotech), LDN-193189 (0.5 μM; Cayman Chemicals) and A83-1 (0.5 μM; Stemgent), CHIR99021 (3 μM; LC Laboratories), Forskolin (5 μM; LC
