SUMMARYComplement testing of aThpline revealed a failure of survival ofThp/t0embryos. The genetic factor responsible for this lethality maps between Brachyury (T) and tufted (tf) on the murine seventeenth chromosome. This lethal factor permits recombination betweenTandtfand does not affect the transmission of its seventeenth chromosome. Its effect upon embryonic development is similar to that of thet0haplotype. It would appear to represent a point mutation of a single gene,t-complex lethal zero(tcl0).