– Scytonemin is a biological pigment synthesized by many strains of cyanobacteria, including Calothrix sp., Lyngbya aestuarii, and others. Scytonemin is also a potent PKL1 inhibitor, which inhibits cell proliferation and arrests cell cycle through downregulating Plk1 activity in many cancel cells. Scytonemin was able to inhibit the proliferation of three myeloma cells in a dose-dependent manner, while U266 was the most sensitive one to scytonemin. Scytonemin, representing a novel Plk1 inhibitor, induced the inhibition of cell growth and cell cycle arrest in multiple myeloma cells by specifically decreasing Plk1 activity.