FAP and breast cancer: Moreover, using the in-house and publicly available primary breast cancer datasets, we observed a significant positive correlation between GREM1 and markers for CAFs and tumor matrix stiffness/desmoplasia, such as FAP, fibronectin (FN)1, fibrillin (FBN)1, collagen (COL)1A1, thrombospondin (THBS)2, and a-actin (ACTA)2 (Fig. 1d, Additional file 2: Figure S1f).