Our previously developed anterograde monosynaptic tracers H129-dTK-tdT and H129-dTK-T2 are derived from H129 and based on TK deficiency, which leads to certain intrinsic drawbacks such as low labeling intensity, low labeling efficiency, and potential retrograde labeling [10, 12, 13]. The gene discussed is TKT; the disease is hyperinsulinemic hypoglycemia, familial, 4.