Heterozygotes are affected on all four feet. Fusion usually occurs between the second and third digits and occasionally involves the fourth (J:13049). The muscles of the forearms and lower legs as well as of the feet show anomalous arrangements not necessarily correlated with the skeletal changes (J:12944). At 11 days of gestation the preaxial border of the limbs can be seen to be reduced (J:12942), and a histological examination at this time shows that there is a small amount of cellular degeneration in the preaxial part of the footplate mesoderm, leading to coalescence of the second and third digital rudiments (J:5107). Os /+ mice have a mild diabetes insipidus present at 5 weeks and increasing with age. In combination with one or more recessive modifying genes in the selected DI stock, Os/+ mice have a severe diabetes insipidus (J:12948). The cause of the diabetes is a 45% reduction in size of the kidneys with an 80% reduction in number of glomeruli. Compensatory hypertrophy of the nephrons is not sufficient to restore normal urine-concentrating ability (J:5127)(J:5128).