(A) Schematic of AAV2-retro-Cre injection. AAV2-retro-Cre is injected into the mystacial pad of Ai14 mice at postnatal day (P)1, P10, P15, or P21. (B) Subnuclei of VII. Each subnucleus is circled by dotted lines. Peripheral muscle targets are shown in each subnucleus. Motoneurons in the lateral part of VII (red) are labeled by AAV2-retro-Cre injection at P21. (C) Labeling patterns from P1, P10, P15, or P21 injected animals. Injection volumes of AAV2-retro-Cre are shown under each panel. Sections were counterstained with fluorescent Nissl (blue).