FARSLB (Phenylalanine--tRNA ligase beta chain), also known as FARSB, FRSB, PheHB or PheRS, is a 589 amino acid cytoplasmic protein that is the regulatory beta subunit of phenylalanine-tRNA synthetase. A highly conserved member of the aminoacyl-tRNA synthetase class IIc subfamily, FARSLB functions to enzymatically attach L-phenylalanine residues to terminal adenosines on tRNA(Phe). This is an ATP-dependent reaction that yields AMP, a diphosphate and an L-phenylalanine-tagged tRNA(Phe). FARSLB is expressed in brain, heart, kidney, pancreas, placenta and skeletal muscle and its expression is increased in malignant cell lines, suggesting a possible role for FARSLB in carcinogenesis.