For clinical and experimental use of iN cells, it would be desirable to develop ways to generate neurons with neurotransmitter- and region-specific phenotypes. Liver- and fibroblast-iN cells generated with the same reprogramming factors displayed properties characteristic of excitatory neurons. While this does not imply the acquisition of a specific region-specific phenotype, this finding could mean that the glutamatergic fate is a default fate as has been suggested for ES cell differentiation systems (Tropepe et al., 2001; Gaspard et al., 2008). Alternatively, the choice of transcription factors could have specifically induced an excitatory subtype. Therefore, the question arises of whether inclusion of subtype-specific transcription factors in the reprogramming cocktail could direct cells into other desired subtypes.
