PDGFRB and cancer: Human triple-negative breast [45,84] and ovarian [85] cancer studies have yielded 3–4 CAF subtypes, designated CAF S1–S4 based on the differential expression of six fibroblast markers (FAP, integrin β1/CD29, α-SMA, S100-A4/FSP1, PDGFRβ, and caveolin-1).