Puromycin is an aminonucleoside antibiotic, derived from S. alboniger, that blocks protein synthesis in both prokaryotes and eukaryotes by causing premature termination of nascent polypeptide chains. A portion of the puromycin molecule resembles the 3" end of aminoacylated tRNA. It enters the A site of the ribosome and attaches to the growing polypeptide chain at the peptidyl-transferase center, causing the formation of a puromycylated nascent chain and premature chain release. The 3" position contains an amide linkage instead of the normal ester linkage of tRNA, making it much more resistant to hydrolysis and halting the activity of the ribosome. These properties of puromycin have been exploited in assays for peptide-bond formation and elongation and also the antibiotic is often used as selective agent in cell culture systems.