The most common routine staining combination. Hematoxylin is used in combination with metal ions, such as aluminum or iron, to form colored chelate complexes. The ingredients of the stain act as cations and are preferentially bound to acidic (anionic) groups in the tissues. The structures to which hematoxylin binds are termed “basophilic” (base-loving). Hematoxylin stains regions rich in nucleic acids blue. Eosin stains many structures (collagen and some areas of cytoplasm) pink. These areas have been termed “acidophilic” but, more properly, should be called eosinophilic.