FAP and esophageal squamous cell carcinoma: Five CAF markers, including platelet-derived growth factor α (PDGFRα), PDGFRβ, smooth muscle actin (SMA), fibroblast activation protein (FAP), and fibroblast-stimulating protein-1 (FSP1), were heterogeneously expressed in stromal fibroblasts of 136 ESCC tissue samples.