In human cell models of nonsense mutations in genes known to cause Batten disease (TPP1 p.L211X and TPP1 p.L527X), Tay–Sachs disease (HEXA p.L273X and HEXA p.L274X) and Niemann–Pick disease type C1 (NPC1 p.Q421X and NPC1 p.Y423X), treatment with the same prime editor programmed to install an optimized sup-tRNA resulted in restoration of 20–70% of normal enzyme activity. This evidence concerns the gene HEXA and Niemann-Pick disease, type C1.