Microcystin-YR is primarily known as a toxic secondary metabolite produced by certain strains of cyanobacteria, especially Microcystis aeruginosa, during algal blooms in freshwater systems. Its main significance lies in environmental and public health concerns due to its potent hepatotoxicity. It inhibits protein phosphatases 1 and 2A in liver cells, leading to cellular damage, hemorrhage, and potential liver failure in humans and animals exposed through contaminated drinking or recreational water. Due to its toxicity, Microcystin-YR serves as a biomarker for cyanobacterial contamination in water quality monitoring. It is used in research settings to study toxin uptake, detoxification mechanisms, and the effects of liver-specific toxins. Additionally, it plays a role in the development and testing of water treatment methods aimed at removing cyanotoxins, such as advanced oxidation, activated carbon filtration, and membrane processes. Because of its stability and toxicity, it is also used as a reference standard in analytical chemistry for detecting and quantifying microcystins in environmental and biological samples using techniques like HPLC and LC-MS.