(Top) Both puromycin and its alkyne-functionalized analog O-propargyl-puromycin incorporate into nascent polypeptide chains on translating ribosomes, resulting in premature termination of nascent polypeptide chains. (Bottom) Sectioning of mouse small intestine showed that OP-Puro labeling occurred primarily in cells in the crypts and the cells at the base of the villi. (Adapted with permission from Liu et al., Proc. Natl. Acad. Sci. U.S.A., 109, 413–418, 2012. Copyright 2012 Proceedings of the National Academy of Sciences USA.)