Asymmetric cyanine compounds represented by general formula I are provided, wherein X, n, R.sub.1, R.sub.2, R.sub.3, R.sub.4 and Y.sup.- are as defined in the specification. They have a maximum absorption peak at about 640 nm which may not change with ambient temperature. When the compounds bind a nucleic acid to form a dye/nucleic acid complex, the fluorescence intensity of the complexes will increased rapidly, so that they can be used as a staining agent for nucleic acids in flow cytometers. Their spectra are in the near-infrared region, which can effectively reduce the interference from background fluorescence and improve the accuracy of detection. Moreover, the compounds provided can also be used as a staining agent for blood reticulocytes. ##STR00001##