We report a mouse model of multiple osteochondromas (MO), an autosomal dominant disease in humans, also known as multiple hereditary exostoses (MHE or HME) and characterized by the formation of cartilage-capped osseous growths projecting from the metaphyses of endochondral bones. The pathogenesis of these osteochondromas has remained unclear. Mice heterozygous for Ext1 or Ext2 , modeling the human genotypes that cause MO, occasionally develop solitary osteochondroma-like structures on ribs [Lin et al. (2000) Dev Biol 224(2):299–311; Stickens et al. (2005) Development 132(22):5055–5068]. Rather than model the germ-line genotype, we modeled the chimeric tissue genotype of somatic loss of heterozygosity (LOH), by conditionally inactivating Ext1 via head-to-head loxP sites and temporally controlled Cre -recombinase in chondrocytes. These mice faithfully recapitulate the human phenotype of multiple metaphyseal osteochondromas. We also confirm homozygous disruption of Ext1 in osteochondroma chondrocytes and their origin in proliferating physeal chondrocytes. These results explain prior modeling failures with the necessity for somatic LOH in a developmentally regulated cell type.