CA9 and breast cancer: The targeting of human epidermal growth factor receptor 2 (HER2) in breast cancers and their metastasis (bone, lung, and lymph nodes), Bhusari et al. (2016), Carbonic Anhydrase IX (CAIX), a protein overexpressed in hypoxic tumor cells, Muselaers et al. (2016), and cholecystokinin2 receptor (CCK2R) in medullary thyroid cancer, Rottenburger et al. (2019), are active areas of research, and listed in Table 2.