Familial clustering is a common feature of many different arthritic conditions which can be explained by shared genetic or environmental influences or a combination of these. The aetiology of some of these diseases (e.g. multiple epiphyseal dysplasia, Lesch-Nyan syndrome) is clearly monogenic but the majority of common rheumatic conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA) are complex and multi-factorial with a polygenic component. In the last 10 years there has been ...