β-Amanitin, a cyclic peptide toxin found in the poisonous Amanita phalloides mushroom, inhibits eukaryotic RNA polymerase II and III, thereby disrupting protein synthesis.

In MCF-7 cells, β-Amanitin (0.01-100 μg/mL; 36 hours) shows toxicity, and the rates of cell viability are calculated as 52%, 62%, 84%, 86%, and 91% at concentrations of 100, 10, 1, 0.1, and 0.01 μg/mL, respectively[2].In MCF-7 cells, β-Amanitin shows a great inhibition of protein synthesis at both concentrations (10 μg/mL and 1 μg/mL) for 24 hours[2].