According to the current monoallelic hypothesis, phenotype is determined solely by the causative allele product. If a second, different allele is present, it is considered a functional null. There is a lack of any correlation between the site of the XPD mutation and the resulting disorder. We propose a biallelic hypothesis for compound heterozygotes in which both alleles can contribute to the phenotype. Examples of compound heterozygous patients in which a second, presumed null allele is likely to contribute to disease outcome are provided above in comparison to corresponding homo- or hemizygous patients with the same causative allele. Numbers in the schematic of the protein indicate the helicase domains.