NEDD8 modified with fluorescein via primary amine coupling is ideal as an alternative to radiolabeled NEDD8. This results in multiple fluoresceinated NEDD8 species with modified lysines as well as the N-terminus. The ubiquitin-like protein NEDD8 is conjugated to targets by the NEDD8-specific E1 activating enzyme (AppBp1/Uba3), the UbcH12 E2 enzyme, and the ROC1/Rbx1 RING FINGER E3 ligase. NEDD8 plays a critical regulatory role in cell proliferation and development, and modifies nearly all members of the cullin family.