Probes for other types of hydrolase enzymes, such as esterases and thioesterases, contain enzyme-cleavable ester and thioester functional groups respectively.49–51 A similar functional group cleavage activation mechanism is observed for aminopeptidases, and this approach was used by Peng and Yoon to develop a CD13/aminopeptidases (APN) N-activatable fluorescent probe for tracking metastatic cancer (Fig. 1B). Here, ANPEP is linked to metastatic malignant neoplasm.