a, Experimental rationale. b, Uninfected, p3 TauEGFP MEFs contained rare Tuj1-positive cells (red) with flat morphology. Blue: DAPI counterstain. c, Tuj1-positive fibroblasts do not express visible TauEGFP. d–e, MEF-iN cells express Tuj1 (red) and TauEGFP (green) and display complex neuronal morphologies 32 days after infection with the 19-factor (19F) pool. f, Tuj1 expression in MEFs 13 days after infection with the 5F pool. g–j, MEF-derived Tuj1-positive iN cells co-express the pan-neuronal markers TauEGFP (h), NeuN (red,i) and MAP2 (red,j). k, Representative traces of membrane potential responding to step depolarization by current injection (lower panel). Membrane potential was current-clamped at around −65 mV. l, Representative traces of whole-cell currents in voltage-clamp mode, cell was held at −70 mV, step depolarization from −90 mV to 60 mV at 10 mV interval were delivered (lower panel). Insert showing Na+currents. m, Spontaneous action potentials (AP) recorded from a 5F MEF-iN cell 8 days post infection. No current injection was applied. n–p, 22 days post-infection 5F MEF-iN cells expressed synapsin (red,n) and vesicular glutamate transporter 1 (vGLUT1) (red,o) or GABA (p). Scale bars = 5 μm (o), 10 μm (e,n,p) 20 μm (c,h,i), and 200 μm (f).