Earlier genetic studies on this stock of mice with an X-autosome translocation revealed that wild-type alleles of pink-eye and chinchilla of linkage group I have been translocated to the X. When the Xt was accompanied by an Xn, animals heterozygous for pp and cchcch showed variegated coat colors but the Xt alone led only to the wild-type coat color. The cytological investigation reported herein revealed the following information: 1. The translocation was non-reciprocal. About one third of the autosome representing linkage group I was transposed into the X, making the Xt about 20% longer than the Xn. 2. The autosomal insertion behaved as an integral part of the X, manifesting positive heteropyenosis when the X itself assumed this condition. 3. In the skin of animals with a variegated phenotype, the cc patches were apparently populated by cells containing a condensed Xt, while in wild-type patches from the same animal, the Xt was isopyenotic, behaving in the same manner as the euchromatic autosomes. These findings support Lyon’s hypothesis explaining the somatic variegation in Mus musculus due to X-autosome translocation (1961, 1962). The present study on skin cells of XtXn females has yielded convincing evidence that the mammalian female is indeed a natural mosaic.