We report a variety of biallelic effects on
organismal phenotype attributable to combinations of recessive Xpd alleles, including the following: (i) the ability of homozygous lethal Xpd alleles to ameliorate a variety of disease symptoms when their essential basal transcription function is supplied by a different disease - causing allele, (ii) differential developmental and tissue - specific functions of distinct Xpd allele products, and (iii) interallelic complementation, a
phenomenon rarely reported at clinically relevant loci in mammals.