Fibroblast-like synoviocytes (FLS) in the intimal lining of the joint synovium control the composition of the synovial fluid and extracellular matrix (ECM) of the joint lining. In rheumatoid arthritis (RA), FLS become aggressive and invasive, contributing to many aspects of RA pathology. FLS produce matrix metalloproteinases (MMPs) that break down the ECM, directly invade and digest the articular cartilage, promote bone erosion, and promote inflammation through secretion of interleukin 6 (IL-6), chemokines, and other inflammatory mediators. FLS are highly sensitive to the inflammatory environment present in rheumatoid joints. Growth factors, especially platelet-derived growvth factor (PDGF), stimulate FLS invasiveness. Inflammatory cytokines, particularly tumor necrosis factor-alpha (TNF) and interleukin-I (IL-1), enhance FLS aggressiveness, pro-inflammatory features and MMP production. Targeting of molecules that control FLS invasiveness and inflammatory output is being considered an option for development of new therapies for RA. Many signaling pathways controlling FLS behavior rely upon phosphorylation of proteins on tyrosine residues, which results from the balanced action of protein tyrosine kinases (PTKs) and phosphatases (PTPs). We found that a protein (PTPRA) belonging to a novel and currently untapped class of drug targets is present at high levels in cells lining the joints of RA patients, where we believe it promotes the aggressive behavior of these cells in joint inflammation and destruction.