The development of transgenic technology has made possible the generation of targeted gene-mutated mouse lines suitable for use in experimental osteoarthritis (OA) research. Transgenic mice harbouring mutations in cartilage collagen types II and IX develop early-onset OA and are therefore promising models of age-related OA, even though the mice often show signs of chondrodysplasia. Also, mouse lines harbouring other engineered mutations of the extracellular molecules have given rise to early OA. The molecular background of a few spontaneous mutations in mice has also been clarified and the characterization of the OA phenotype is now in progress. These mutations cause severe chondrodysplasia and death in homozygous mice, but the heterozygous offspring develop the early-onset OA phenotype.