Analysis was performed with the statistical software package Prism Origin (GraphPad Software). Statistical analysis of iMN survival experiments was performed using a 2-sided log-rank test to account for events that did not occur (i.e., iMNs that did not degenerate before the end of the experiment). For each line, the survival data from 90 iMNs were selected randomly using Microsoft Excel, and these data were used to generate the survival curve. If all iMNs degenerated in a given experiment, statistical significance was calculated using a 2-tailed Student’s t test.