FAP and intervertebral disk degenerative disorder: Contractile myofibroblasts are primary effector cells of tissue fibrosis.8 Fibroblast activation protein-α (FAPα, encoded by FAP) and FSP1 (encoded by S100A4) mark the active fibroblasts or myofibroblast precursors,17 and αSMA (encoded by ACTA2) marks the mature myofibroblasts.18 These three markers, which were used to indicate fibroblastic NPC in mouse IDD,4,9 were found enriched in the FibroNP (Fig. 2a).