Illudin S is a potent antitumor sesquiterpene metabolically activated into reactive intermediates that cause a complete block of cell cycling at the G1-S phase interface, particularly in myeloid and T-lymphocyte leukemia cells (IC50 = 6-11 nM). It was first isolated from the fungus Clitocybe illudens and disrupts thymidine-dependent DNA synthesis without affecting RNA or protein synthesis.