Abstract We recently published a report demonstrating that, remarkably, adult corticospinal neurons in the mouse revert to an embryonic transcriptional state after spinal cord injury (SCI) (Poplawski et al. Nature 2020). This state persists for two weeks, then reverts to an adult transcriptional state in which neurons are more refractory to regenerative therapies. This grant seeks to determine whether embryonic reversion of corticospinal neurons also occurs in primate corticospinal motor neurons after SCI, and if so, how long this state endures. This is critical information for planning human clinical trials because efforts to repair the injured spinal cord are most likely to be effective in the time period that corticospinal are in an embryonic state and most able to regenerate and respond to pro-growth therapies. Our results to date have shown that embryonic reversion of primate corticospinal neurons indeed appears to occur. We sampled time points of 3, 10 and 20 days after SCI. We find that embryonic reversion is present at Day 10 and is nearly gone by Day 20. We now seek to add an additional time point, 15 days, to this study. If embryonic reversion is still present 15 days after SCI, one could extend the duration of potential patient recruitment into clinical trials, representing an important improvement in increase the number of people eligible for pro-regenerative trials. This information is important not only for our stem cell program that we aim to clinically translate, but for any pro-regenerative treatment trial in humans. We will use techniques that have been successfully employed in this grant to date. We will perform RNAseq of corticospinal neurons in rhesus monkeys using retro-AAV vectors expressing HA- and FLAG- tagged polyribosomes. We then use translational Ribosomal Affinity Purification (TRAP) to obtain and sequence corticospinal-specific mRNA species. We will add to our current timeline a 15 days post injury group (n= 3) and one more monkey (n=1) to our 10 day post top strengthen statistical power and enable batch correction.