Rare recessive diseases, like Behr's syndrome and MTRFR deficiency stemming from MTRFR variants, tend to be good candidates for gene replacement therapies such as adeno-associated virus (AAV) delivery of rescuing wild-type complementary DNAs (cDNAs) (Di Meo et al., 2017; Hanaford et al., 2022). The gene discussed is MTRFR; the disease is Behr syndrome.