XER can stand for:

Xer site-specific recombination in vitro.
The dif/Xer recombination systems in proteobacteria.
Xer-mediated site-specific recombination at cer generates Holliday junctions in vivo
Topological selectivity in Xer site-specific recombination.
FtsK-dependent and -independent pathways of Xer site-specific recombination.
Decatenation of DNA circles by FtsK-dependent Xer site-specific recombination.
Direct interaction of aminopeptidase A with recombination site DNA in Xer site‐specific recombination
The ArcA/ArcB two-component regulatory system of Escherichia coli is essential for Xer site-specific recombination at psi
Peptidase activity of Escherichia coli aminopeptidase A is not required for its role in Xer site‐specific recombination
X-ray structure of aminopeptidase A from Escherichia coli and a model for the nucleoprotein complex in Xer site-specific recombination