Rothmund-Thomson syndrome (RTS; also known as poikiloderma congenitale) is a rare, autosomal recessive genetic disorder characterized by abnormalities in skin and skeleton, juvenile cataracts, premature ageing and a predisposition to neoplasia 1, 2, 3, 4. Cytogenetic studies indicate that cells from affected patients show genomic instability often associated with chromosomal rearrangements causing an acquired somatic mosaicism 5, 6, 7, 8, 9. The gene (s) responsible for RTS remains unknown. The genes responsible for Werner 10 and Bloom 11 syndromes (WRN and BLM, respectively) have been identified as homologues of Escherichia coli RecQ, which encodes a DNA helicase 12 that unwinds double-stranded DNA into single-stranded DNAs. Other eukaryotic homologues thus far identified are human RECQL (Refs 13, 14), Saccharomyces cerevisiae SGS1 (Refs 15, 16) and Schizosaccharomyces pombe rqh1+(ref. 17). We recently cloned two new human helicase genes, RECQL4 at 8q24. 3 and RECQL5 at 17q25, which encode members of the RecQ helicase family 18. Here, we report that three RTS patients carried two types of compound heterozygous mutations in RECQL4. The fact that the mutated alleles were inherited from the parents in one affected family and were not found in ethnically matched controls suggests that mutation of RECQL4 at human chromosome 8q24. 3 is responsible for at least some cases of RTS.