Trichothiodystrophy (TTD) is a rare, autosomal recessive neurodevelopmental disorder most commonly caused by mutations in ERCC2 (XPD), a gene that encodes a subunit of the transcription/repair factor IIH (TFIIH). Here, we describe two TTD cases in which detailed biochemical and molecular investigations offered a clue to explain their moderately affected phenotype. Patient TTD22PV showed new mutated XPD alleles: one contains a nonsense mutation (c.1984C>T) encoding a nonfunctional truncated product (p.Gln662X) whereas the second carries a genomic deletion (c.2191-18_c.2213del) that affects the splicing of intron 22 and generates multiple out-of-frame transcripts from codon 731. XPD mRNA from the second allele corresponds to 20% of the total. The predicted proteins, which are longer than normal, affect the cellular repair activity but only partially interfere with TFIIH stability, suggesting that the observed changes in the C-ter region of XPD cause minor structural changes that do not drastically compromise the transcriptional activity of TFIIH. Patient TTD24PV was compound heterozygous for a typical TTD allele (c.2164C>T, p.Arg722Trp) and for a new XPD allele with a mutation that partially affects intron 10 splicing, resulting in both mutated and normal XPD transcripts (that together represent 15% of the total XPD mRNA). Compared to the previously described TTD compound heterozygotes for the Arg722Trp change, Patient TTD24PV's cells show similar level of TFIIH but increased repair activity, suggesting that even low amounts of normal XPD subunits are able to partially rescue the functionality of TFIIH complexes. Hum Mutat 0, 1–8, 2008. © 2008 Wiley-Liss, Inc.