Repetitive extragenic palindromic (REP) sequences are highly conserved inverted repeats present in up to 1000 copies on the Escherichia coli chromosome. We have shown both in vivo and in vitro that REP sequences can stabilize upstream mRNA by blocking the processive action of 3'----5' exonucleases. In a number of operons, mRNA stabilization by REP sequences plays an important role in the control of gene expression. Furthermore, differential mRNA stability mediated by the REP sequences can be responsible for differential gene expression within polycistronic operons. Despite the key role of REP sequences in mRNA stability and gene expression in a number of operons, several lines of evidence suggest that this is unlikely to be the primary reason for the exceptionally high degree of sequence conservation between REP sequences. Other possible functions for REP sequences are discussed. We propose that REP sequences may be a prokaryotic equivalent of 'selfish DNA' and that gene conversion may play a role in the evolution and maintenance of REP sequences.