To recapitulate the photosensitive form of trichothiodystrophy, part of exon 22 and all of exon 23 were replaced with a fragment of human cDNA, containing an arginine to tryptophan missense mutation. Northern blot analysis of total testicular RNA showed overexpression of the mutant transcript. Protein level analysis was not possible due to the lack of an antibody that detects the hybrid protein.