On day 14, iPSC-derived microglia (approximately 30,000 cells per coverslip; table S1) were added into the Ngn2-iNs using fresh Neurobasal medium containing B-27 supplement, l-glutamine, BDNF, NT3, GDNF, IL-34, and GM-CSF. Molecular, morphological, and functional analyses were conducted approximately 5 weeks after the initial induction with doxycycline.