Fluorescent labels or probes are ideal for many imaging and diagnostic applications. Fluorescent groups can be covalently connected to the N-terminal or C-terminal, although it is recommended to bind to the N-terminal. FRET pairs composed of receptors and quenching agents can be connected internally or externally. For longer sequences, it is recommended that you connect FRET pairs internally. Fluorescent labeled peptides have been used to detect the activity of target proteins and their developed high throughput activity screening methods have been applied to drug screening and drug development of disease treatment target proteins (for example, Various kinases, phosphatases, peptidases, etc.). Therefore, the fluorescence modification of peptides is also an important content in the field of drug development.