CD133, also known as prominin-1, is a pentaspan transmembrane glycoprotein encoded by the PROM1 gene. It is widely used as a biomarker for the identification and isolation of stem and progenitor cells in various normal tissues and cancer stem cells (CSCs) in different types of tumors. CD133 has five transmembrane domains, with its N-terminus extending into the extracellular space and the C-terminus residing inside the cell. It is expressed in neural progenitor cells, hematopoietic cells, and various cancer cells, including those in brain, colon, and pancreatic tumors. The presence of CD133 is associated with self-renewal, tumorigenicity, and resistance to chemotherapy and radiotherapy, making it a significant marker for cancer prognosis and therapeutic targeting.