Nucleic Acids Research, 2006, Vol. 34, No. 22 6661 Figure 9. Prothrombin time assessed by using human plasma. On the x-axis the incubation time is reported, while PT time in seconds is reported on y-axis. (A) Refers to TBA, while (B) to 1. is not to be considered an anomaly, but is rather a robust G- quadruplex scaffold (23–26). The structure reported here rep- resents also the ﬁrst example of a ‘3+1’ core topology quadruplex structure containing only two G-tetrads. Further- more, our ﬁndings may contribute to the comprehension of the folding of telomeric DNA (23–26), and could shed more light on the understanding of the thrombin/TBA interac- tion phenomenon. Finally, considering that the ‘3+1’ arrange- ment increases the thermal stability of the quadruplex and that 1 is not devoid of biological activity, the scaffold described in the present paper can be seen as a unique platform for structure-based anticoagulant drug design. 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