Toyocamycin is a nucleoside- type antibiotic analogue of adenosine, isolated from Streptomyces species. Toyocamycin is an antibiotic first isolated by Nishimura et al. from a new species of Streptomyces (St. toyocaensis), with a rather specific antibiotic spectrum. It strongly inhibits Candida albicans and Mycobacterium tuberculosis, without notable action on other microorganisms, such as many gram-positive and gramnegative bacteria, fungi, and yeast. Toyocamycin is an anti-tumor antibiotic with various target activities. Toyocamycin is a potent inhibitor of RNA self-cleavage in mammalian cells. It also inhibits phosphatidylinositol kinase, a cell proliferation regulator. Toyocamycin can suppress thapsigargin-, tunicamycin- and 2-deoxyglucose-induced XBP1 mRNA splicing in HeLa cells. This suppression doesn’t affect the activating of transcription factor 6 (ATF6) and PKR-like ER kinase (PERK)’s activation. Toyocamycin prevents IRE1a-induced XBP1 mRNA cleavage in vitro. In mammalian cells, toyocamycin inhibits RNA synthesis. Toyocamycin induces apoptosis of MM cells including bortezomib-resistant cells at nanomolar levels in a dose-dependent manner. It also inhibited growth of xenografts in an in vivo model of human multiple myeloma. It is also a lead compound for developing anti-MM therapy and XBP1 as an appropriate molecular target for anti-multiple myeloma therapy. Toyocamycin was the first identified small-molecule inhibitor of Rio1, showing mixed inhibition. This mode of action of toyocamycin results from its dual activity towards the Rio1 kinase. On the one hand, toyocamycin acts as an ATP-competitive inhibitor, and on the other hand, it stabilizes the less catalytically active oligomeric isoform of the Rio1 kinase.