a–d PAHenu2 mice (n = 6–16/treatment group) were treated with a single IV dose of mRNA-3210, and serum (n = 9 mice for 0.25 mg/kg mRNA-3210 at 0.25 h postdose and n = 8 mice for all other timepoints; n = 9 mice for 0.5 mg/kg mRNA-3210 at 0.5 h postdose and n = 8 mice for all other timepoints; n = 9 mice for 1.0 mg/kg mRNA-3210 at 0.25 h postdose and n = 8 mice for all other timepoints) and liver hPAH mRNA (n = 9 mice for 0.25 mg/kg mRNA-3210 at 2 h postdose, n = 8 mice at 8, 24, 48, 96 h postdose, n = 8 mice at 8 h postdose, and n = 16 mice at 168 h postdose; n = 8 mice for 0.5 mg/kg mRNA-3210 at 2, 24, 48, 96 h postdose, n = 9 mice at 8 h postdose, and n = 16 mice at 168 h postdose; n = 9 mice for 1.0 mg/kg mRNA-3210 at 2 h postdose, n = 8 mice at 8, 24, 48, 96 h postdose, n = 8 mice at 8 h postdose, and n = 16 mice at 168 h postdose) or hPAH protein concentrations (n = 10 mice for 0.25 mg/kg mRNA-3210 at 2 h postdose, n = 8 mice at 8, 24, 48, 96 h postdose, n = 7 mice at 96 h postdose, and n = 16 mice at 168 h postdose; n = 8 mice for 0.5 mg/kg mRNA-3210 at 2, 24, 48, 96 h postdose, n = 9 mice at 8 h postdose, and n = 16 mice at 168 h postdose; n = 9 mice for 1.0 mg/kg mRNA-3210 at 2 h postdose, n = 8 mice at 8, 24, 48, 96 h postdose, n = 16 mice at 168 h postdose), were assessed at baseline and 6, 24, 48, 72, 96, and 168 h postdose. d PAHenu2 mice (n = 8/treatment group) were treated with a single IV dose of mRNA-3210 at 0.25, 0.5, or 1.0 mg/kg or saline (control). Blood Phe levels were assessed at baseline and 6, 24, 48, 72, 96, and 168 h postdose. enu2 N-ethyl-N-nitrosourea, hPAH human phenylalanine hydroxylase, IV intravenous, mRNA messenger RNA, PAH phenylalanine hydroxylase, Phe phenylalanine, PKU phenylketonuria, SD standard deviation. Source data are provided as a Source Data file.