Interleukin 34 (IL34) is a hematopoietic cytokine that acts as a key regulator of survival, proliferation and differentiation of myeloid lineage cells including monocytes, macrophages and osteoclasts. IL34 was identified in 2008 in a comprehensive proteomic analysis as a tissue-specific ligand of CSF-1 receptor (CSF-1R). This cytokine shares numerous common features with M-CSF, especially its receptor (MCSF-R) explaining partly their functional overlap. IL34 is expressed in various tissues and it is most abundant in the spleen. Upon inflammation, other cells, such as fibroblasts and articular synovial cells, upregulate IL34 expression. Recent studies have reported that IL34 levels are elevated in the serum, synovial fluid, and synovial tissue of RA patients, and like CSF-1, IL34 is induced by TNF and IL-1beta in RA fibroblast-like synoviocytes (FLS).