TTD-specific cutaneous and anaemic features, on the other hand, are thought to result from a specific kind of transcriptional insufficiency caused by depletion of unstable TFIIH during the terminal differentiation of skin, hair-shaft, and blood cells [16,24]. In compound heterozygous mice, both homozygous lethal Xpd†XPCS and Xpd†XP alleles were able to alleviate XpdTTD-specific cutaneous and anaemic features and can thus be defined as dominant over the XpdTTD allele with respect to these phenotypes. We conclude that the defects leading to embryonic lethality and aberrant terminal differentiation of the skin, hair, and blood represent two qualitatively and/or quantitatively different transcriptional deficiencies. During early embryonic development, XpdTTD is dominant over the Xpd†XPCS and Xpd†XP alleles, whereas later in the ontogenesis of skin, hair-shaft, and blood cells, the situation is reversed.