TCO-GK-PEG4-NHS ester is an ADC Linker , and can be used for synthesis of [18F] AlF-NOTA-Tz-TCO-GK-2Rs15d. [18F] AlF-NOTA-Tz-TCO-GK-2Rs15d binds with high affinity and immunoreactivity to HER2. TCO-GK-PEG4-NHS ester is a click chemistry reagent, it contains a TCO group that can undergo an inverse electron demand Diels-Alder reaction (iEDDA) with molecules containing Tetrazine groups.