early embryo, such as OCT4 and NANOG, thus granting the appropriate TFs access to the promoter (Raab et al., 2014). Within ~ 5 weeks in culture, a small percentage (≤ 1%) of the initial somatic cells are reprogrammed to pluripotent stem cells, which are an infinite source of starting material from which several defined cell types may be generated and investigated (Kim, Choi, Choi, & Do, 2011). Patient-derived iPS cell lines can then be used to model human neurological disease by differentiating them into tissue-specific cells germane to the disease of interest. To comprehensively model human neurodegenerative and neuropsychiatric disorders, many efforts have been committed to generating defined neuronal cell types from iPS and ES cells (Marchetto, Brennand, Boyer, & Gage, 2011). However, iPS cells may not fully recapitulate the epigenetic landscape of the individuals (Mertens, Paquola, et al., 2015), but direct-differentiated induced neuronal (iN) cells by ectopic expression of neuronal lineage TFs (Pang et al., 2011) can then serve to better elucidate the molecular mechanisms underlying neurological disorders.
