Moving p38-EGFP–containing vesicles do not contain an endosomal marker. Eight-day-old neurons were transfected with p38-EGFP cDNA, labeled 22 h later for 1–2 h with 1 mg/ml Rh-dextran, and analyzed by two-color video microscopy as described in MATERIALS AND METHODS. Top, consecutive frames showing p38-EGFP-fluorescence; bottom, corresponding frames showing Rh-dextran fluorescence. Arrowheads point to moving p38-EGFP–containing vesicles; arrows point to a moving Rh-dextran–containing vesicle. Asterisks mark nonmoving structures that contain both markes. Note that moving vesicles contain one marker only. Bar, 10 μm.