In contrast, the level of XPCS mRNA expression did affect the ability of the encoded protein (XPDG602D) to restore the TTD hair phenotype to normal. Notably, XpdTTD/ †XPCS animals had a partial TTD hair phenotype, correlating with low levels of Xpd†XPCS expression, whereas XpdTTD/XPCS animals had wt hair, correlating with normal expression levels from the viable XpdXPCS allele (Table 2 and unpublished data). Thus, the range of expression levels from these two mutant alleles affected their ability to complement some phenotypes (hair). An overview of the functional relationships between Xpd alleles, phenotypes, and the presumed underlying TFIIH function in mice and cells is presented in Table 2.