Myo1b controls the size of the F-actin–enriched area in the growth cones. (A) Cortical neurons were transfected with control or Myo1b-siRNAs, immunolabeled for Myo1b and β3-tubulin, and fluorescently labeled for F-actin at DIV2. Representative confocal z stacks for Myo1b, merged confocal z stacks for F-actin (green) and β3-tubulin (magenta), and images of the 3× enlargements of the regions marked by the white boxes are shown. (B) The measurements of the area occupied by F-actin in the growth cone of the longest neurite as judged by β3-tubulin labeling in cells transfected with Myo1b- or control siRNAs are represented as box plots. n = 51 in three independent experiments. (C) Cortical neurons expressing EGFP, EGFP-Myo1b, or EGFP-Myo1b-K966A, immunolabeled for β3-tubulin, and fluorescently labeled for F-actin at DIV2. Representative merged confocal z stacks for EGFP (green), β3-tubulin (magenta), and F-actin (red) and images of the 2.5× enlargements of the regions marked by the white boxes are shown. Bars, 20 µm. (D) The measurements of the area occupied by F-actin in the growth cone of the longest neurite as judged by β3-tubulin labeling in cells expressing EGFP, EGFP-Myo1b, or EGFP-Myo1b-K966A are represented as box plots. n = 25 in three independent experiments. Data distribution was assumed to be normal. Unpaired t test. *, P < 0.05; **, P < 0.01; ***, P < 0.001.