Chondrodysplasia (cho/cho) is a recessive disorder in mice that affects cartilage important in the embryogenesis of cochlea. These mice show marked hearing loss when tested by auditory brain-stem responses. The temporal bone shows underdevelopment of the organs of Corti in the lower turn of the cochlea. Also, there are no supporting cells, inner or outer hair cells, nerve endings, or pillar cells. At the upper part of the cochlea, however, the organ of Corti is almost normal in structure. The exact nature of the molecular genetic abnormality that gives rise to the structural and functional cochlear anomalies seen in these mice is unknown.