The thiol-reactive BODIPY™ FL maleimide produces electronically neutral dye conjugates that are spectrally similar to the negatively charged fluorescein 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™ FL dye has proven useful for studying ligand-receptor interaction by fluorescence polarization. In addition, BODIPY™ FL dye has little or no spectral overlap with longer-wavelength dyes such as tetramethylrhodamine and Texas Red™ dye, making it a useful green fluorophore for multicolor applications.