TRIM9 is expressed in the developing murine cortex. (A) Murine TRIM9 isoforms. COS, C-terminal subgroup one signature. (B–D) Immunoblots (IB) using a TRIM9 polyclonal antibody demonstrate TRIM9 expression in embryonic whole brain lysate (B), in embryonic cortex (C), and in cortical neurons (D) cultured for the indicated times. βIII-Tubulin is a loading control. The data shown in C are shown again in Fig. S2 E alongside TRIM9^−/− samples. (E and E′) Immunohistochemistry (IHC) of E15.5 brain using TRIM9 monoclonal antibody (black) reveals TRIM9 expression in the developing cortex. The dotted square is the ROI shown in E′. (F and F′) EGFP-TRIM9 localizes to punctae at the periphery of the cell body, along the axon shaft (arrow) and at the tips of F-actin–positive filopodia (arrowheads; F′); phalloidin is shown in red. Boxes show the ROI represented in the enlarged color-combined image in F’.