ALK and neoplasm: Fluorescent probes can be generally categorized into three types based on their targets: (i) Tumor antigen probes, such as those that respond to carcinoembryonic antigen (CEA) and cytokeratin 19 fragment (CYFRA21-1); (ii) Tumor-related enzyme probes, including those that respond to neutrophil elastase (NE), aminopeptidase N (APN), β-galactosidase (β-gal), and neuron-specific enolase (NSE); (iii) Membrane receptor protein fluorescent probes, such as those that target epidermal growth factor receptor (EGFR) and anaplastic lymphoma kinase (ALK).