Four 13C- and 15N-labelled SIL (stable isotope labelled) peptides were used in a subsequent experiment to quantify the RCA isoforms in transgenic plants. The peptide sequences used are shown in Supplementary Table S2, with asterisks denoting the position of the labelled amino acids as follows: K∗ = Lys U13C6; U-15N2 and F∗ = Phe U-13C9; U-15N. Each vial of SIL peptides (1 nmol/vial) was suspended in 500 µL of MQ water. The resuspended four SIL peptides were then mixed, resulting a mixed SIL peptide stock solution with a concentration of 0.5 pmol µL−1 for each SIL peptide. 10 µL of this mixed SIL peptide solution was added into each 10 µg rice protein sample. The concentration factor for SIL peptides was 500 pmol mg−1. Further mass spectrometry was performed on the TripleTOF 6600(Sciex)and data analysis as described above.