To determine the extent of rescue of Ia proprioceptive afferent projections into the ventral spinal cord of Er81 mutant mice achieved by expression of EWS-Pea3, we traced intraspinal afferent projections by axonal labeling of PV (Figure 2A–2C). In addition, to analyze axon ingrowth independent of the level of PV expression in DRG neurons, we used anterograde labeling of afferent fibers by applying fluorescently labeled dextran to cut dorsal roots (Figure 2D–2F). Using both assays, we found extensive rescue of the projections into the ventral horn of the spinal cord in Er81EWS-Pea3/− mice (Figure 2C and 2F). Within the ventral horn, Ia afferents in both wild-type and Er81EWS-Pea3/− mice formed vGlut1+ terminals that were absent in Er81 mutant mice (Figure 2G–2I). To assess whether synapses between Ia afferents and motor neurons are functional in Er81EWS-Pea3/− mice, we performed intracellular recordings from identified quadriceps motor neurons after stimulation of nerves innervating the quadriceps muscle group. We found no significant difference in the input amplitude to quadriceps motor neurons when comparing wild-type to Er81EWS-Pea3/− mice (Figure S2; wild-type, 10.6 ± 0.9 mV, n = 11; Er81EWS-Pea3/−, 10.9 ± 1 mV, n = 8). Together, these findings suggest that in the absence of Er81, EWS-Pea3 can direct the complex biological process of correct laminar termination within the ventral spinal cord and the formation of synapses with motor neurons (Figure 2J–2L), thus identifying an ETS transcription factor suitable for heterochronic expression experiments in DRG neurons.