CD34 and rheumatoid arthritis: Angiogenesis is a critical driver of rheumatoid arthritis (RA) development [1], as are endothelial progenitor cells (EPCs) [2,3], which contain the cell surface markers CD34, CD133 and vascular endothelial growth factor receptor 2 (VEGFR2), which stimulate postnatal vasculogenesis [4] and angiogenic function [5].