Used as a stable isotope-labeled tracer in metabolic studies to track phenylalanine metabolism in humans and animals. Helps in diagnosing phenylketonuria (PKU) by measuring metabolic flux and enzyme activity. Employed in pharmaceutical research to study protein synthesis and degradation. Supports development and validation of analytical methods like mass spectrometry and NMR due to its distinct isotopic signature. Also utilized in labeled protein production for structural biology studies.