A novel mobile element system, which appears to exploit the chromosomal XerC/XerD site-specific recombination system to mobilise discrete DNA segments, was first discovered in Acinetobacter baumannii plasmids over a decade ago, but has received little attention until recently. Bioinformatic studies have revealed that a number of genes that confer resistance to carbapenem antibiotics, tetracycline or macrolides are located between sites that resemble the chromosomal dif site that is located near the terminus and acted on by XerCD to resolve chromosomal dimers. The sites, now called pdif sites (plasmid dif sites), bound the discrete mobile units which are called dif modules and are most often found in small Rep_3 plasmids. In addition, various toxin-antitoxin genes potentially involved in plasmid stability are in dif modules as are genes potentially involved in virulence or resistance to environmental assaults such as ethanol tolerance or metabolic genes. The system appears to be predominantly confined to species within the Acinetobacter genus and has been found in Acinetobacter species recovered from permafrost samples, indicating a long association with this genus. The XerC and XerD binding sites in the 28 bp pdif sites resemble those of the dif site but the 6 bp spacer between them is highly variable and differs from the dif spacer. Each dif module is surrounded by pdif sites in inverse orientation, either a D/C with C/D (C modules) or C/D with D/C (D modules) and they occur in groups where C modules alternate with D modules. Recombination between inversely-oriented sites would lead to inversion and between directly-oriented sites to excision. Recently, we have experimentally demonstrated both inversion and excision (loss of two adjacent modules) in a single plasmid. However, in the XerCD-dif system, spacer sequence identity in reacting pairs is important for efficient recombination, and, likewise, these reactions only occurred between sites with identical spacers. How single dif modules surrounded by inversely-oriented sites can move remains to be established.