The invention relates to orthogonal pairs of tRNAs and aminoacyl-tRNA synthetases that can incorporate unnatural amino acids into proteins in mammalian host cells, for example, primate host cells and rodent host cells. The invention provides, for example but not limited to, translation systems that include host cells (e.g., primate or rodent cells), orthogonal aminoacyl-tRNA synthetases derived from eubacterial synthetases, orthogonal tRNAs, and the unnatural amino acid. The invention also relates to methods for producing proteins of interest comprising at least one unnatural amino acid in mammalian host cell systems.