A common endpoint for a biomarker discovery experiment is a list of putative marker proteins. The next step is then to perform targeted quantitative measurements of these proteins in an expanded patient population to assess their validity as markers. Analytical accuracy and precision are required for unambiguous quantitative analysis of targeted proteins from very complex mixtures. Wide dynamic range and high sensitivity are critical for detecting low-abundance proteins. Such an assay also is appropriate for "targeted discovery" experiments, where the goal is to quantitate a large number (up to hundreds) of known proteins in a complex sample.