PDGFRB and breast cancer: Costa et al. divided breast cancer (BC) CAFs into four subsets S1-S4 (Table 1) based on the expression of fibroblast activation protein (FAP), caveolin-1 (CAV-1), CD29, fibroblast specific protein 1 (FSP-1), PDGFRβ, and alpha-smooth muscle actin (αSMA).