Female heterozygotes of the shaking pup and myelin-deficient rat sex-linked recessive traits, show myelin mosaicism of the optic nerve and spinal cord. This is most marked in the optic nerve especially in the rat where mosaic patches persist with aging. In both the rat and dog, abnormal oligodendrocytes with distended rough endoplasmic reticulum are found in the abnormal patches and are a marker of the trait. Female heterozygote dogs can develop a marked tremor which disappears with age.