Since dietary supplementation of creatine leads to the attenuation of symptoms in AGAT (AGAT-D) and GAMT deficiency (GAMTD-D, [47,48]), and fails to restore creatine levels in CRT deficiency (CRT-D) [27], our polymeric nanovectors might represent an important tool to i) optimize the biodistribution and half-life of creatine in AGAT-D and GAMTD-D patients; ii) devise an effective therapeutic strategy for CRT-D. The gene discussed is GATM; the disease is guanidinoacetate methyltransferase deficiency.