FAQ: Is dam-/dcm- (NEB #C2925H/C2925I) resistant to kanamycin?

NEB #C2925H/C2925I has a low but noticeable frequency of kanamycin resistant mutations. We have observed a low, variable number of spontaneous Kan resistance mutants in this strain, and also in another commercially available dam-/dcm- strains (Invitrogen INV110). In both strains, tiny colonies appear on Kan plates at a frequency ranging from 1 in 10e6 to 1 in 10e7 cells. These tiny colonies will not grow upon restreaking on a fresh Kan plate. This level of background resistance should not be a problem for routine transformation of plasmids carrying KanR. The reason for the strain-dependent appearance of incomplete KanR is not known.