Abstract: Disclosed is a class of reagents for examining protein expression in vivo that does not require transfection, radiolabeling, or the prior choice of a candidate gene. Further, a series of puromycin conjugates was constructed bearing various labeling moieties. These conjugates were readily incorporated into expressed protein products in cell lysates in vitro and efficiently cross cell membranes to function in protein synthesis in vivo as indicated by flow cytometry, selective enrichment studies, and western analysis. The present invention demonstrates that labeled-puromycin conjugates offer a general means to examine protein expression in vivo.