N3,N3,N6,N6-Tetramethylacridine-3,6-diamine, also known as acridine orange base, is a heterocyclic aromatic compound featuring an acridine core symmetrically substituted with two dimethylamino groups. It is a yellow to orange crystalline powder with a molecular formula of C17H19N3. This compound is primarily recognized as the free base form of Acridine Orange, a well-known metachromatic fluorescent dye. It functions as a versatile nucleic acid stain, capable of intercalating into double-stranded DNA (emitting green fluorescence) and stacking with single-stranded RNA or denatured DNA (emitting red fluorescence). This property makes it a crucial tool in cytogenetics, flow cytometry, and fluorescence microscopy for cell cycle analysis, chromosome staining, and the detection of apoptotic cells. Beyond its direct use as a dye precursor, the molecule's structure, comprising a planar acridine ring and tertiary amine functionalities, makes it a valuable building block in organic synthesis, particularly for constructing larger π-conjugated systems or as a ligand precursor. It requires careful handling due to its potential mutagenicity and should be protected from light during storage.