SNARE proteins are required for cortical axon branching. (A) E15.5 TRIM9^+/+ cortical neurons were untreated or Netrin-1 stimulated, with the addition of 10 nM BoNTA, 10 nM TeNT, and/or longin expression (blue), and stained with fluorescent phalloidin (red) and for βIII-tubulin (green). Arrowheads denote branch points. (B) Mean axon branch density ± SEM. (C) E15.5 TRIM9^−/− cortical neurons were untreated or Netrin-1 stimulated, with the addition of 10 nM BoNTA, and stained with fluorescent phalloidin (red) and for βIII-tubulin (green). Arrowheads denote branch points. (D) Mean axon branch density ± SEM. Red asterisks in B and D denote statistical significance from TRIM9^+/+ neurons in the absence of Netrin-1 (P < 0.5). Black asterisks denote statistically significant differences between untreated and Netrin-1–stimulated conditions (P < 0.5). All graphs are at least n > 54 neurons/condition from more than three independent experiments (n = 1,353 neurons). P-values are from ANOVA with Tukey’s post-hoc analysis.