The protocol is illustrated in Extended Data Fig. 2a. Specifically, iPS cell colonies were detached from the feeder layer with 1 mg ml−1 collagenase treatment for 1h and suspended in embryoid body (EB) medium, consisting of FGF-2-free iPS cell medium supplemented with 2 μM dorsomorphin and 2 μM A-83, in non-treated polystyrene plates for 4 days with a daily medium change. After 4 days, EB medium was replaced by neural induction medium (hNPC medium) consisting of DMEM/F12, N2 supplement, NEAA, 2mg ml−1 heparin and 2 μM cyclopamine. The floating EBs were then transferred to Matrigel-coated 6-well plates at day 7 to form neural tube-like rosettes. The attached rosettes were kept for 15days with hNPC medium change every other day. On day 22, the rosettes were picked mechanically and transferred to low attachment plates (Corning) in hNPC medium containing B27. For neuronal differentiation, resuspended neural progenitor spheres were dissociated with Accutase at 37°C for 10 min and placed onto poly-D-lysine/laminin-coated coverslips in the neuronal culture medium, consisting of Neurobasal medium supplemented with 2 mM L-glutamine, B27, 10 ng ml−1 BDNF and
