PTPN11 and systemic sclerosis: Double staining with the fibroblast marker prolyl-4-hydroxylase-β (P4Hβ), the endothelial marker CD31 and the leukocyte marker CD45 demonstrated that fibroblasts account for most of the SHP2 expression in the dermis and that SSc fibroblasts express reduced levels of SHP2 compared to fibroblasts in healthy skin (Fig. 1c).