The invention relates generally to peptide biomarkers with specific ionization characteristics to directly quantify one or more transgenic target proteins in biological samples, including transgenic plant samples, by liquid chromatography coupled tandem mass spectrometry multiple reaction monitoring (MRM). The peptide biomarkers in combination with MRM-based methods may be used to quantify a single transgenic target protein or multiple transgenic target proteins within a stacked transgenic crop, such as maize, utilizing selected peptide biomarkers either alone or in combination. The present disclosure allows for broad based, reliable quantitation in different biological matrices, including plant matrices. The peptide biomarkers of the invention can further be used as trait biomarkers to support identification and/or selection of specific transgenic Events. Also provided are different peptide biomarker combinations that can be used to perform the methods of the invention.

The invention relates to methods for quantitative multiplex analysis of complex protein samples from plants using mass spectroscopy. In some embodiments, the disclosure concerns methods for maintaining a transgenic plant variety, for example by analyzing generations of a transgenic plant variety for selective and sensitive quantitation of multiplexed transgenic proteins.