XL2-Blue MRF´ ultracompetent cells are high-efficiency derivatives of our XL1-Blue MRF´ supercompetent cells.  Using ultracompetent cell technology, we have achieved transformation efficiencies of greater than 5 × 10⁹ transformants/µg of pUC18 DNA, making these ultracompetent cells an ideal choice for high-efficiency cloning. The XL2-Blue MRF´ (Minus Restriction) strain is deficient in all known restriction systems (McrA–, McrCB–, McrF–, Mrr–, HsdR–), preventing cleavage of cloned DNA that carries cytosine and/or adenine methylation, which is often present in eukaryotic DNA and cDNA.   XL2-Blue MRF´ cells are restriction minus. The strain is endonuclease deficient (endA), greatly improving the quality of miniprep DNA, and recombination deficient (recA), helping to ensure insert stability. These cells allow blue-white screening for recombinant plasmids.