The CYS21 strain is recommended for cloning applications when using StabyCloning, StabyExpress, CherryExpress, StabyCodon, CherryCodon and GetStaby. This strain was derived from standard E. coli K12 cloning strain. It contains the ccdB selection gene in its chromosome allowing plasmid stabilization and selection of recombinants. This selection system is an antibiotic-free system.   

The genotype of the CYS21 strain is: F-, CmR, mcrA, endA1, Δ(mrr-hsdRMS-mcrBC) (restriction-, modification-), Φ80lacZΔM15, ΔlacX74, recA1, Δ(ara, leu)7697, araD139, galU, galK, nupG, rpsl, ccdB+.

CYS21 bacteria are available either as electro- or chemically- competent cells. Bacteria are shipped (in dry ice) with SOC medium. Each tube of CYS21 electro-competent cells contains 50ul of competent bacteria (>1X10⁹ cfu/ug DNA ). Each tube of CYS21 chemically-competent cells contains 100ul of comptetent bacteria (>1x10⁸ cfu/ug DNA).(Other forms of packaging are available on request).