A fluorescence-labelled tracer, that can bind to PKMYT1, and the N-terminal GST-tagged full-length human PKMYT1 are used in this assay kit. A Terbium-labeled anti-GST antibody binding to the GST-PKMYT1 serves as a fluorescence donor (HTRF donor). If the fluorescence-labeled tracer binds to the PKMYT1, the binding brings Terbium on the anti-GST antibody close to the fluorophore on the tracer (HTRF acceptor). Activation of the Terbium results in fluorescence resonance energy transfer (FRET). Thus, the binding status can be quantitively measured by calculating the ratio of the emission fluorescence intensity of the acceptor (665 nm) and donor (620nm). The competitive binding of a non-fluorescence-labeled compound will reduce the FRET signal.