We observed differences in the ability of XpdTTD versus homozygous lethal Xpd†XPCS and Xpd†XP alleles to function in two transcription-related phenotypes separated in the organism by both time and space: embryonic lethality and terminal differentiation of enucleating skin and blood cells. The preblastocyst-stage homozygous lethality shared by the XpdKO, Xpd†XPCS, and Xpd†XP alleles most likely reflects a defect in basal transcription that is incompatible with life. In XpdTTD/ †XPCS and XpdTTD/ †XP compound heterozygous mice, embryonic lethality was fully rescued by the XpdTTD allele. Because embryonic lethality was also fully rescued in XpdTTD/KO hemizygous mice, the XpdTTD allele can be considered as wt and thus dominant to each of the homozygous lethal alleles (XpdKO, Xpd†XPCS, and Xpd†XP) with respect to this particular phenotype (Table 2).