The gene of β -galactosidase from E. coli is widely used as a reporter gene assay marker. Although X-gal is a well known reagent to detect β -galactosidase in cell or tissue samples, the assay using these reagents require to fix cells or tissues due to the poor cell-permeability. In addition, the assay using fluorescence reagents can not clearly differentiate β -galactosidase - expressed cells or regions in its current development. SPiDER-Gal ideally possesses cell-permeability and the ability to retain in the intracellular region. With the enzymatic reaction, SPiDER-Gal immediately forms a quinone methide that acts as electrophile when proteins containing nucleophilic functional groups are nearby the molecules. When the probe undergoes the reaction with a protein, the conjugates become fluorescent compounds. SPiDER-βGal allows a single-cell analysis because it self-immobilizes to the intracellular proteins.