Protein quantification and quantitative assessment of multiple post-translational modifications (PTM) within a single protein are becoming increasingly important in the biomarker discovery, validation, and proteomic studies. Mass spectrometry (MS) combined with in vitro protein expression system and target-specific antibody enables a novel quantitative strategy for such applications. Specifically, a full-length expressed stable isotope-labeled protein generated from a eukaryotic wheat germ system is used as an internal standard in MS to track and quantify not only the protein of interest but also all the PTM within a single protein. The robust in vitro wheat germ system overcomes the laborious, inefficient, and low yield process in protein standard production. Moreover, this MS-based quantification method only requires sub-microgram amounts of protein standard for analysis. When an antibody for the protein of interest is available, it even allows quantification of low-abundance proteins and their PTM. Abnova provides ready-to-use, in vitro wheat germ expression kits and a library of available antibodies covering all the essential protein families to assist you to start and achieve your quantitative needs.