The thiol-reactive BODIPY™ TMR maleimide produces electronically neutral dye conjugates that are spectrally similar to the positively charged tetramethylrhodamine dye. This dye's lack of ionic charge results in minimal effects on the isoelectric points of standard proteins conjugated with this fluorophore. The small size and relatively long excited-state lifetime of BODIPY™ TMR dye has proven useful for studying ligand-receptor interaction by fluorescence polarization.