Fluorescein (resorcinolphthalein, Figure 1) is a fluorescent dye consisting of two isomers, 5(6)-carboxyfluorescein, linked to the spirolactam ring system, one of which is 6-Fam (6-carboxyfluorescein), a most commonly used dye; it is most efficiently quenched by BHQ-1.
Figure 1. Structure of 5'-Fluorescein (R = sequence).
metabion offers the Fluorescein modification in its DNA and RNA portfolio: