Sodium 4-Phenylbutyrate-d11 is intended for use as an internal standard for the quantification of sodium 4-phenylbutyrate by GC- or LC-MS. Sodium 4-phenylbutyrate is a chemical chaperone that has been shown to rescue the trafficking of misfolded proteins. It also weakly blocks histone deacetylase activity (IC50 = 0.4 mM), which results in cell cycle arrest, differentiation, and/or apoptosis of various tumors. Formulations containing sodium 4-phenylbutyrate have been used for the treatment of urea cycle disorders.