Affect quantification accuracy in some cell types, we investigated arginine to proline conversion in KM12 cells. By adding heavy proline as a variable modification, less than 2 of proline-containing peptides were heavy labeled in KM12 cells, which has been reported not to affect significantly SILAC quantification (25). Western Blot--For Western blot, conditioned media from KM12C and KM12SM CRC ce