The development of two or more cartilage capped bony outgrowths of the long bones.Develop and increase in size in the first decade of life, ceasing to grow when the growth plates close at puberty. The number may vary significantly within and between families. The majority are asymptomatic and located in bones that develop from cartilage, especially the long bones of the extremities, predominantly around the knee. The most important complication is malignant transformation towards secondary peripheral chondrosarcoma. Germline mutations in EXT1 or EXT2, are found in almost 90% of patients. An autosomal dominant disorder.