Following our initial observation that the combined expression of Brn2, Ascl1, and Myt1l induces functional neurons from human ES cells (Pang et al., 2011), we examined whether forced expression of a series of single transcription factors in ES and iPS cells might initiate iN cell differentiation. As in previous studies (Vierbuchen et al., 2010; Pang et al., 2011), we used lentiviral delivery for constitutive expression of rtTA (Urlinger et al., 2000), and tetracycline-inducible expression of exogenous proteins driven by a tetO promoter. Surprisingly, we found that overexpressing either neurogenin-2 (Ngn2) or NeuroD1 alone rapidly converted ES and iPS cells into neuronal cells (Figs. 1 and S1). Since this conversion was based on forced expression of a lineage-specific transcription factor and appears to be a direct lineage conversion similar to lineage conversion between somatic cells, we refer to the resulting neurons as iN cells as previously (Vierbuchen et al., 2010; Pang et al., 2011).
