NEUROD1 and infection: Next, NR-astrocytes were infected once (× 1) or three times (× 3) with the set of the three viruses (EGFP-, M2rtta-, and NeuroD1-viruses) (Fig. 2B), and we found that the repeated infection increased both NeuroD1 and EGFP protein expression levels at the single-cell level (Fig. 2C,D).