ACTA1 and systemic sclerosis: Kimberly Showalter [41] mentions smooth muscle actin as a key marker in the cutaneous histology of SSc, as it is a protein expressed by myofibroblasts, cells involved in the cutaneous fibrosis characteristic of the disease, and it has been found that α-SMA levels in skin biopsies correlate with the severity of systemic sclerosis and with the response to treatment in clinical trials of antifibrotic drugs.