For standard cloning experiments competent cells are available in two formats: high efficiency at greater than 108cfu/μg for cloning (JM109, HB101), and subcloning efficiency at greater than 107cfu/μg (HB101). JM109 cells have the additional feature of blue/white screening for recombinants.
For protein expression, JM109(DE3), BL21(DE3) or KRX competent cells can be used. The Single Step (KRX) Competent Cells are designed for efficient transformation and tightly controlled protein expression. These cells consolidate the best attributes of these two steps into one strain to evaluate protein expression in E. coli.