We found a new X-linked dominant mouse mutation. This mouse has the same phenotype as Td, which exhibits hyperkeratotic skin, reduced viability in affected females, a tendency to be smaller, lighter weight than the normal sibs during weaning age, and prenatal lethality in affected males. To map the locus, we tested 267 progeny from an intraspecific backcross between affected females and wild-origin strain males. Polymerase chain reaction (PCR) was performed with microsatellite markers of the proximal region of the mouse X Chromosome (Chr). This mutant showed no recombination with DXMit 123, DXMit 55, or DXMit 26. The gene position and phenotype of this mutant were very similar to those of Td. Therefore, it is speculated that the new mutant gene is a multiple allele of Td, and we designated it Tattered-Hokkaido (Tdho). Linkage analysis of these animals suggested a possible gene order of cen-(Tdho, DXMit123, DXMit55, DXMit26)-DXMit161-DXMit54-DXMit103-DXMit52 -DXMit190-DXMit138) in the X Chr. Prenatal lethality of male mutants was also investigated, with 12.5 to 16.5 embryonic day (E) backcrossed embryos from affected F1 females. It was found that the male mutants died between E12.5 and E14.5. The cause of death of male mutants is discussed in relation with the other proximal genes of the X Chr.