PAJaneliaFluor646, SE (PA-JF646-NHS), a photoactivatable fluorescent dye, is an NHS ester for coupling to primary amine groups. PA-JF646-NHS is non-fluorescent until activated at 365 nm. NHS ester can be converted to relevant substrate for use in self-labeling tag systems, e.g.HaloTag and SNAP-tag . PA-JF646-NHS is used for single molecule tracking and super resolution microscopy in live cells, specifically live cell sptPALM and fixed cell PALM. (λ Excitation emission ~650 664 nm).