- xerB, an Escherichia coli gene required for plasmid ColE1 site-specific recombination, is identical to pepA, encoding aminopeptidase A, a protein with substantial similarity to bovine lens leucine aminopeptidase. Stirling, C.J., Colloms, S.D., Collins, J.F., Szatmari, G., Sherratt, D.J. EMBO J. (1989)