Because patients with TTD, XPCS, and CS (but not XP) and the corresponding mouse models share similar accelerated progeroid symptoms [12,13,15,23], we next addressed ageing-related parameters in compound heterozygous mice (Figure 3). Whereas XpdTTD/TTD animals show reduced bone mineral density as an indication of the early onset of osteoporosis before ~14 mo of age [15], tail vertebrae from compound heterozygous XpdTTD/†XPCS mice were comparable to wt even at 20 mo of age (Figure 3B and 3C). Furthermore, whereas XpdTTD/TTD mice developed kyphosis earlier than wt animals (onset ~3 mo versus 12–20 mo), compound heterozygous XpdTTD/†XPCS mice did not (Figure 3B). Overall appearance and body weight curves revealed that TTD-associated age-related premature cachexia and lack of general fitness were fully rescued in compound heterozygous XpdTTD/†XPCS mice (Figure 3A and 3D). Finally, the life span of compound heterozygotes was extended relative to XpdTTD/TTD mice (Table 2).