Next we asked whether the Xpd†XPCS and Xpd†XP alleles, despite decreased mRNA levels, ameliorated TTD symptoms by increasing overall TFIIH levels in compound heterozygous XpdTTD/ †XPCS and XpdTTD/ †XP cells. Previously, using comparative immunohistochemistry, we and others have shown an up to 70% reduction of TFIIH levels in primary fibroblasts from patients with TTD compared with wt controls due to reduced stability [16,17]. Despite overexpression of mRNA from the XpdTTD allele relative to the wt allele (Figure 1E), TFIIH protein levels were reduced by 50% in primary mouse XpdTTD/TTD fibroblasts (Figure 4E and 4F), thereby mimicking the situation in human patients with TTD. In accordance with the gene dosage, a further reduction of up to 70% of the wt level was observed in hemizygous XpdTTD/KO cells. Consistent with low mRNA expression levels, neither the Xpd†XPCS nor the Xpd†XP allele was able to restore TFIIH abundance to wt levels in XpdTTD compound heterozygote cells (Figure 4E and 4F). Thus, the improved UV survival observed in compound heterozygote cells (Figure 4A) and likely the rescue of TTD progeroid symptoms (Figure 3) were not due to normalisation of TFIIH levels, suggesting a qualitative rather than a quantitative effect on these phenotypes in vivo.