Taking the temperature.

of nicked DNA substrates. This finding could be of therapeutic importance. Dideoxynucleoside drugs and their analogues — such as ddC, ddI, AZT, D4T and T3C — are not usually incorporated into genomic DNA in large amounts. But they do get into our genome through the action of polymerase b, which can incorporate them into repair patches during base excision repair 8 , and through another polymerase (g), which inserts them into mitochondrial DNA 9. Both of these events should cause problems, as incorporating these substances into DNA generates strand breaks that cannot be joined. Thus, by removing them from genomic DNA, APE1 helps our cells to survive. In contrast, because DNA synthesis by HIV takes place in the cytoplasm, where there is no APE1, chain terminators incorporated into the proviral DNA by the HIV enzyme reverse transcriptase are not removed. As a result, synthesis of full-length HIV DNA can be inhibited. So, by improving the efficiency of APE1-mediated removal of chain termina-tors from genomic and mitochondrial DNA, it should be possible to reduce the side effects of HIV therapy. The APE1-catalysed removal of nucleo-side analogues from DNA termini is also interesting from the structural point of view. All of these substances form standard Watson–Crick pairs with the nucleosides in the complementary strand. This implies that, rather than seeing the mispairs as such, APE1 may be detecting subtle distortions of the double helix at the nick, characterized by misalignments of the strand ends that make it impossible to rejoin them. Gorillas that have learned to use sign language remain a rarity, even though they pop up on television from time to time. The newly discovered proofreading APEs, by contrast, are clearly commonplace and, in contrast to signing gorillas, likely to be of considerable clinical importance. ■ The deep 'overturning' circulation in the North Atlantic, in which northward-flowing surface water sinks at high latitudes and flows back at depth, may not be fully understood, but its existence has long been public knowledge. The Pacific has held on to some of its secrets for longer. Only in the 1990s were two shallow overturning circulation cells discovered, lying on either side of the Equator. As they describe elsewhere in this issue have now produced an analysis of historical data to show that both cells have been slowing since the 1970s. The circulation loops occur because winds produce surface currents away from the Equator. Water …