The primary use of hematoxylin in cancer diagnosis is in the Hematoxylin and Eosin (H&E) stain. This is the most common staining technique used in histopathology. It helps pathologists to distinguish between normal and abnormal tissue architecture. Hematoxylin stains cell nuclei blue, allowing for the identification of mitotic figures and the assessment of nuclear morphology, which are critical in diagnosing various types of cancer.