RNA non-speciﬁcally, it does recognize other sequences or structural motifs in RNA in addition to tightly binding to G4-RNA. These preliminary observations suggest a sig- niﬁcantly more complicated binding interaction with RNA than DNA. These G4R1/RHAU-RNA binding interactions are currently under further investigation in our laboratory. Our data suggest that it is most likely that G4R1/ RHAU interacts with both DNA and RNA in cells. Although DNA interaction may be statistically less common per protein binding event, it may still potentially be of great biological importance. The amplifying nature of control of G4-DNA promoters or the control of telomere structures could have profound biological effects. If the enzyme does not act upon DNA, one would have to speculate the existence of special cellular mechanisms to exclude interaction with DNA. This study of G4R1/RHAU interactions with unimolecular G4-DNA highlights the ability of this enzyme to recognize and process the unimolecular G4-DNA structure. In the case of binding to G4-DNA, G4R1/RHAU exhibits extraor- dinarily tight binding afﬁnity for G4-DNA, and the cap- ability of directly destabilizing G4 structures, thereby enabling the G4 forming nucleotide sequence to be avail- able for Watson–Crick type duplex hybridization. The data in this report open up a number of unimolecular G4-DNA sequence-containing targets as potential sub- strates for G4R1/RHAU. One can speculate that G4R1/ RHAU might have direct activity at substrates such as the human telomere, G-rich promoters and G-rich nontranscribed strands of active genes. These potential targets of action suggest interesting future experiments to test the biological actions of this enzyme. SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. ACKNOWLEDGEMENTS We would like to thank Terrence Oas of the Duke University Medical Center for kindly allowing us to use their AVIV 202 CD spectrometer and Yu-Chu Chang for technical assistance with it. We thank Mark Lively of Wake Forest Health Sciences for helpful comments on the manuscript. We thank Nancy Maizels of University of Washington Medical Center for allowing us to use her G4-DNA schematics. FUNDING The National Institutes of Health (Grant P30-CA12197) and a gift from the Ware Foundation (to J.V.). National Institutes of Health Institutional Training (Grant T32-CA079448) (to B.G.). Funding for open access charge: A gift from the Sara Self Foundation. Conﬂict of interest statement. None declared. Nucleic Acids Research, 2011, Vol. 39, No. 16 7177 REFERENCES 1. Gellert,M., Lipsett,M.N. and Davies,D.R. (1962) Helix formation by guanylic acid. Proc. Natl Acad. Sci. USA, 48, 2013–2018. 2. Macaya,R.F., Schultze,P., Smith,F.W., Roe,J.A. and Feigon,J. 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