A label with which labeling is easy when labeling a molecule, i.e., a label that has a high reaction rate upon labeling and that produces a high reaction yield, as well as a precursor for the production of the label are provided. This is achieved by a hexatriene-β-carbonyl compound represented by Formula (I), a hexatriene-β-carbonyl compound represented by Formula (II), a hexatriene-β-carbonyl compound represented by Formula (III) and a hexatriene-β-carbonyl compound represented by Formula (IV).(wherein R1, R2, R3, R4, R5, X1, X2 and Z are as defined in the specification)
