NeuroD1 [27]. More recently another study claimed the generation of iN cells from embryonic fibroblasts without use of bHLH factors by simple knock-down of the splicing factor PTB [28]. Those mouse iN cells surprisingly exhibited both spontaneous glutamatergic and GABAergic postsynaptic events in the absence of primary neuronal co-culture. This finding implicates the presence of glutamatergic and GABAergic subtypes of iN cells. Since only one single gene was knocked-down in this experiment, it appears that the subtype specification was not controlled by this genetic manipulation and was stochastic. This conclusion would have important mechanistic implications on how neuronal subtypes actually develop under normal differentiation conditions.
