The murine model for Turner Syndrome is the XO mouse. Unlike their human counterparts, XO mice are typically fertile, and their lack of a second sex chromosome can be transmitted from one generation to the next as an X-linked dominant trait with male lethality. The introduction of an X-linked coat-color marker (tabby) has greatly facilitated the maintenance of this useful mouse strain. XO mice can be produced in large numbers, generation after generation, and rapidly identified on the basis of their sex and coat color. Although this breeding scheme appears to be effective at the phenotype level, its utility has never been conclusively proved at the molecular or cytogenetic levels. Here, we clone and sequence the tabby deletion break point and present a multiplex polymerase chain reaction-based assay for the tabby mutation. By combining the results of this assay with whole-chromosome painting data, we demonstrate that genotype, phenotype, and karyotype all show perfect correlation in the publicly available XO breeding stock. This work lays the foundation for the use of this strain to study Turner Syndrome in particular and the X chromosome in general.