The hallmark of rheumatoid arthritis (RA) is progressive destruction of the joints, preceded and accompanied by synovial hyperplasia and chronic inflammation. Spontaneous and induced animal models of RA reflect predominantly the inflammatory aspects of the disease. To reproduce the destruction of cartilage and bone mediated by an activated synovium, it was desirable to develop models that allow the dissection of cellular and molecular components derived from human tissue. The SCID mouse co-implantation model of human RA focuses on RA synovial fibroblasts (RA-SF) and their role in cartilage destruction. The model has provided the best evidence that RA-SF contribute significantly to matrix degradation, even in the absence of human lymphocytes and macrophages, since highly purified RA-SF invade the co-implanted normal human cartilage. Moreover, it became clear that they maintained their aggressive phenotype over long periods of time, particularly at sites of invasion into the co-implanted human cartilage. Targeting different signaling molecules, cytokines and matrix-degrading enzymes by soluble receptors, antagonists or negative mutants in the SCID mouse model of RA has implicated many of them in the mechanisms leading to cartilage destruction. However, since inhibition of a single molecule or pathway is not sufficient to inhibit the aggressive behavior of RA-SF it appears necessary to co-express in the synoviocytes genes for two or even more antagonists of e.g. cytokines, matrix-degrading enzymes or molecules interfering specifically with signaling pathways involved in the apoptosis of RA-SF. Based on the recent observation that the L1 (line-1) endogenous retroviral element appears responsible for the cytokine- independent activation via the MAPK p38delta, the current understanding of disease pathogenesis suggests that both the cytokine-dependent as well as the cytokine-independent pathways of joint destruction must be inhibited. Modulation of both pathways by gene transfer approaches in the SCID mouse model is a feasible method aimed at identifying novel targets for the prevention of cartilage destruction in RA.