Disclosed herein are compositions comprising blocked puromycin analogs that are converted into active puromycin analogs upon the activity of a penicillin acylase. Also disclosed are methods of using blocked puromycin analogs to label proteins in a selected cell type in vivo in a transgenic multicellular organism that expresses a penicillin acylase within the selected cell type. Also disclosed are transgenic mice expressing a penicillin acylase within a selected cell type.