AbstractCraniomyeloschisis (proposed gene symbol cms) was a spontaneous mutation of the rat inherited as an autosomal recessive trait with complete penetrance. Homozygous offspring died at birth with failure of closure of the neural tube caudal to the midbrain. Neural crest derivatives were relatively normal. There were associated severe malformations of the axial skeleton including skull, vertebral column, and ribs. The pattern of malformations was quite uniform. Heterozygotes were anatomically normal. The mutation is extinct.