To provide a stable squarylium
pigment derivative which exhibits low
cytotoxicity and safety, and which enables selective and efficient
staining of mitochondria contained in eukaryotic cells, particularly live eukaryotic cells, a mitochondrial
staining reagent comprising the squarylium
pigment derivative, and a method for
staining mitochondria using the squarylium
pigment derivative.SOLUTION: The present invention provides a squarylium pigment derivative or a salt thereof, represented by the following formula (I). [In the formula, R1 and R2 independently represent a C1-6
alkyl group, a C6-12
aryl group, or a C6-12 arylmethyl group; R3 and R4 independently represent a
single bond or a
linker group; and Z1 and Z2 independently represent H or a cationic group, with at least one of Z1 and Z2 representing a cationic group].SELECTED DRAWING: None