The genetic information stored in DNA is read out every minute. That information is transcribed into RNA, and then into protein to fulfill various biological functions. Meanwhile, the DNA in each cell of our body is suffering from all kinds of damages: chemical bonds breaking and forming, DNA strands snapping, nucleotide bases flying off. If these damages were to persist, mutations would be induced and accumulate and the chances of survival would be reduced. Fortunately, the DNA repair enzymes, a well-trained crew, take care of this maintenance job. For many years DNA repair and transcription had been considered fully separable processes. Recently this concept has been challenged by several discoveries that the transcribed DNA strand is preferentially repaired compared with non-transcribed DNA strand in expressed genes (Mellon et al., 1987; Mellon and Hanawalt, 1989; Hanawalt et al., 1994; Bohr et al., 1985).  These findings led to the idea that transcription could be coupled to the selective repair of the transcribed strand.  It's unsurprising that evolution has supported a mechanism for coupling transcription and repair.   It is beneficial to organisms to be able to repair active genes that code for essential proteins.  It has been suggested that this mechanism is particularly important in higher eukaryotes (van Gool et al., 1994).  This might be due to a higher rate of global genome repair in prokaryotes compared to humans (Mellon and Hanawalt, 1989; Tu, Tornaletti, and Pfeifer, 1996).