Palindromic units (PU or REP) were initially defined as a DNA sequence of 40 nucleotides which is highly repeated in the genome of several enterobacteria and found in clusters of up to six copies. It appears now that PU belong to a larger repeated DNA element, of up to 300 nucleotides, called BIME for bacterial interspersed mosaic element. BIME is a mosaic combination of ten small DNA motifs, including the PU sequence. A central question concerning BIME is to determine whether they play a critical role within the cell. BIME exhibit only limited effects on local gene expression; it seems unlikely that these weak effects alone can account for the high BIME sequence homogeneity. It has recently been shown that DNA gyrase and DNA polymerase I are able to specifically recognize BIME DNA in vitro. These findings suggest that BIME could play a role in the functional organization of the bacterial nucleoid. Hypotheses on their origin and evolution are discussed.