S100A4 and breast neoplasm: Although CAFs in breast tumors have been traditionally identified using the marker alpha-smooth muscle actin, CAFs are a heterogeneous population of cells that can be separated into subsets based on the expression of markers including fibroblast activation protein (FAP), fibroblast-specific protein-1 (FSP-1/S100A4), platelet-derived growth factor alpha (PDGFα) and PDGFβ [61–64].