DDR2 and myalgic encephalomeyelitis/chronic fatigue syndrome: Additionally, some markers such as α-SMA, discoidin domain-containing receptor 2 (DDR2), fibroblast-specific protein 1 (FSP1), stem-cell antigen 1 (Sca1) and vimentin were initially used to identify CFs; however, these were not unique markers of fibroblasts and were also not expressed by the whole CF population [13,14,15,16].