Puromycin is an aminonucleoside antibiotic isolated from Streptomyces alboniger. Puromycin terminates protein synthesis at the level of translation in prokaryotic and eukaryotic cells and is active against gram-positive bacteria and weakly active against gram-negative bacteria. Resistance to puromycin is conferred by the puromycin N-acetyl-transferase (pac) gene [1, 2]. Exogenous expression of the pac gene is commonly used in conjunction with puromycin to generate stable cell lines. Most working concentrations range from 1-10 μg/ml; however, the optimal concentration should be determined for individual cell types and culture conditions. In most cases, puromycin kills 99% of non-resistant cells within 48 hours.