[C.I. 52005]. C14H14N3SCl; asymmetrical dimethylthionine chloride; a blue dye used as a component of MacNeal’s tetrachrome blood stain and of Romanowsky-type blood stains; also used as a stain for mucins, nucleic acids, and mast cell granules; gives a metachromatic violet to red color to highly acidic substances in tissues.