Dear Editor,In recent years,the proteomics field has been transformed from charting static proteomes to examining their dynamics by simultaneously quantifying multiple proteins from different experimental samples.Accurate quantification of differences in protein expression levels is essential for comparative proteomics and is becoming an integral and important part of mode experimental biological science (Simicevic et al.,2013).Advanced mass spectrometry (MS) provides a powerful means for achieving quantitative proteomics data,and the preferred approach is to use metabolic labeling with stable isotopes (Gouw et al.,2010).In metabolic labeling,heavy stable isotope labels are introduced in vivo by growing cells or entire organisms in the presence of amino acids or nutrients carrying these isotopes.Introducing the heavy isotope label results in a predictable mass difference between a labeled peptide and its unlabeled counterpart,and relative protein quantification is obtained by comparing the signal intensities of the unlabeled/labeled peptide pairs of a given protein.The labeled and unlabeled samples are combined prior to sample preparation,thus avoiding sample-to-sample variation that may occur during experimental handling.