isoIndigotin, also known as indirubin, is a red bisindole compound and the cis-isomer of indigotin (indigo). It appears as a red solid and is primarily used in biochemical and pharmaceutical research as a potent inhibitor of cyclin-dependent kinases (CDKs) and glycogen synthase kinase-3 (GSK-3), with applications in studying cell cycle regulation, cancer therapy, and neurodegenerative diseases. Historically, it is a component of the traditional Chinese medicine Danggui Longhui Wan, used for treating chronic myelogenous leukemia. In addition, due to its color properties, it finds niche uses as a red dye in textiles and inks, though far less common than synthetic alternatives. Its high purity (≥98%) makes it suitable for laboratory and industrial specialty applications.