The invention relates to a method for the Cell Type specific labeling with Amino acid Precursors (CTAP). In particular, the disclosed method permits the incorporation of stable isotope-labeled amino acids into the proteome of a vertebrate cell that has been engineered to express an exogenous enzyme that enables the cell to produce an essential amino acid from its amino acid substrate. The method employs stable isotope-labeled amino acid substrate/precursors from which essential amino acids bearing the label are generated. The labeled amino acids generated by the transgenic cell not only supports growth but specifically labels proteins of the transgenic cell. Furthermore, the use of different populations of cells expressing different exogenous amino acid-producing enzymes permits differential labeling of the proteomes of the individual cell populations in multicellular environments.
