Marius Wernig's original induced neuron paper (iN).

Authors started with 19 TFs, subset to 5 that had a large effect on conversion efficiency when removed, then dropped Pou3f4 due to redundancy with Pou3f2. They then began dropping 2 TFs at a time from the 5 TF pool and found Ascl1, Pou3f2, Mytl1 was the most efficient, with Pou3f2, Ascl1, Zic1 also improving on the 5 TF efficiency.