Interestingly, a study characterizing BM hematopoietic progenitor cells in 16 refractory RA patients who were candidates for autologous stem cell transplantation, identified risk factors for impaired mobilization and engraftment.46 The authors observed alterations in RA bone marrow including markedly reduced cellularity, lower CD34 + cells and higher apoptotic index in myeloid precursors with respect to healthy controls. Here, CD34 is linked to rheumatoid arthritis.