To determine whether the homozygous lethal Xpd†XPCS allele was unique in its ability to ameliorate symptoms associated with the XpdTTD allele, we generated compound heterozygous XpdTTD/†XP mice by crossing the corresponding heterozygous animals. Similar to the Xpd †XPCS allele, the homozygous lethal Xpd †XP allele rescued cutaneous symptoms including hair loss (except locally during the first round; unpublished data), reduced cysteine content (cysteine index 9.3 ± 0.9 standard deviation [87% of wt], p = 0.01 versus TTD), ageing-associated premature cachexia (males and females were 36.1 ± 6.4 g [93% of wt] and 39.2 ± 3.2 g [116% of wt], respectively), and reduced life span (Table 2). Taken together, these data indicate that two independent alleles, which on their own are unable to support viability (Table 1), were nonetheless able to ameliorate TTD-associated phenotypes in vivo (Table 2).