The ExAssist helper phage and the R408 helper phage are supplied in
7% dimethylsulfoxide (DMSO) and should be stored at –80°C. The helper
phage may be stored for short periods of time at –20°C or 4°C. It is
important to titer the helper phage prior to each use. Expect titers of
approximately 1010 pfu/ml for the ExAssist helper phage or 1010 pfu/ml for
the R408 helper phage. If the titer drops over time, prepare a fresh high-titer
stock of the helper phage as outlined in Amplifying the Helper Phage.