Fibroblast activation protein (FAP), a type II trans-membrane serine protease, is associated with the extracellular matrix of the tumor microenvironment and is highly expressed in cancer-associated fibroblasts (CAFs), which are a subpopulation of fibroblasts in stromal cells with critical roles in tumorigenesis and metastasis [13,14,15,16]. Here, FAP is linked to cancer.