Nuclear Fast Red is a synthetic azo dye widely used in histology and pathology as a nuclear counterstain. It stains cell nuclei red, providing clear visualization under microscopy, and is often used in combination with other stains like Alcian blue or in the Von Kossa method to highlight calcium deposits in tissues. This makes it valuable for diagnosing conditions such as arterial calcification or kidney stones. Additionally, in bone histology, it helps visualize osteoid seams and mineralized bone, aiding studies of bone formation and remodeling. Its selective binding to nucleic acids and calcium-related structures positions it as an essential tool in research and clinical diagnostics.