Lentiviruses were produced in HEK293T cells as described (Zhang et al., 2013). In short, HEK293T cells were transfected using Ca2+-phosphate in T75 flasks with a given lentiviral plasmid and the three helper plasmids pRSV-REV, pMDLg/pRRE, and VSV-G. The culture medium was harvested 40–44 hours after transfection, and virus particles were pelleted by ultra-centrifugation. The viral pellet was resuspended in MEM (100 µl per starting dish) and aliquots of the viral solution were snap-frozen in liquid nitrogen for storage. For viral transduction, 2–5 µl of lentiviral solutions were added to the culture medium of 6-well plate (for H1 ES cells) or a 24-well plate (for human neuronal cultures). Lentiviruses were produced from the following plasmids: i. Lentiviruses to trans-differentiate ES cells into neurons (TetO-Ngn2-P2A-puromycin and rtTA; Fig. S1A; Zhang et al., 2013); ii. Lentiviruses for overexpression of DLK for rescue (pLX304-DLK; clone ID: HsCD00413295), of MBIP for inhibiting DLK (pLX304-MBIP; HsCD00420627), and of MKK7 (pCW45-MKK7; HsCD00298961); for all lentiviral plasmids were obtained from Harvard Medical School; iii, Lentiviruses for overexpression of dominant-negative cFos (DN cFos, Olive et al., 1997)(cDNA from Addgene
