The present invention provides, among other things, methods of treating ornithine transcarbamylase (OTC) deficiency, including administering to a subject in need of treatment a composition comprising an mRNA encoding an ornithine transcarbamylase (OTC) protein at an effective dose and an administration interval such that at least one symptom or feature of the OTC deficiency is reduced in intensity, severity, or frequency or has delayed onset. In some embodiments, the mRNA is encapsulated in a liposome comprising one or more cationic lipids, one or more non-cationic lipids, one or more cholesterol-based lipids and one or more PEG-modified lipids.
