Precise and accurate protein quantification is critical to many areas of proteomics. Antibody-based approaches are costly and time-consuming to develop, consequently, there is considerable interest in alternative quantitative methods that are versatile and can be implemented without the considerable delays associated with antibody development and characterization. Approaches based on MS have therefore attracted considerable attention and are now frequently touted as the most practical and powerful of all options. Nevertheless, there are serious limitations associated with quantifying a protein based on tandem mass analysis of one or two peptides generated by either chemical or enzymatic cleavage. In an accompanying Viewpoint article, Molloy and coworkers point out that selectivity is not necessarily guaranteed despite the power of SRM. Here we address an additional concern that can also compromise specificity. In complex mammalian systems, multiple proteins can serve as precursors of a single peptide and consequently, depending on the peptide(s) selected, protein levels may be significantly under- or overestimated.