(a) Diagram of a transverse sectional view of mouse tail lateral veins and ventral artery. (b) We developed a simple mouse holder made from common laboratory items that allows hydrodynamic injection. (c) Place the holder horizontally. Find the tail vein. Entry of the needle is approximately at a 20-30゜angle. As the needle is inserted into the vein, the needle becomes more parallel with the tail and the solution is injected rapidly between 5 and 8 sec.