In BC, CAFs are heterogeneous and divide into 4 population groups: CAF-S1 to CAF-S4, according to differential activation marker expression mainly including α-smooth muscle actin (ASMA), fibroblast activation protein (FAP), PDGF receptor β (PDGFRβ), fibroblast-specific protein-1 (FSP-1), caveolin-1 (CAV-1), and CD29, with all correlating with poor prognosis (Table 1). Here, FAP is linked to breast cancer.