In its role in the repair of UV photolesions, the Xpd†XPCS allele imparted a clear UV survival benefit over a single XpdTTD allele or two XpdXPCS alleles independent of expression levels, which is consistent with interallelic complementation. However, the observation that no other cellular or biochemical UV-related parameters were improved in XpdTTD/ †XPCS argues against complementation of this repair activity in the rescue of TTD progeroid symptoms in vivo.