Cycloheximide
 
C₁₅H₂₃NO₄ (Fig. 1; molecular mass, 281.35). Cycloheximide is an antibiotic produced by Streptomyces griseus, and it inhibits protein biosynthesis in eukaryotes. Cycloheximide does not affect protein synthesis in prokaryotes or in mitochondria. It binds to the 60S subunit of eukaryotic ribosomes and inhibits the peptidyl transferase activity. The inhibition of peptidyl transfer causes a stack of ribosomes on the messenger RNA, generating polysome accumulation in cells. Cycloheximide is frequently used for the study of gene expression in eukaryotes as an inhibitor of protein synthesis in vivo, as well as in vitro.

Figure 1. Cycloheximide.