Switchable IgM+àIgA+ murine CH12-F3 Ly-1+ B cell lymphoma line was maintained and passaged every 2 days in RPMI 1640 supplemented with 10% FBS (ATCC), 1% penicillin/streptomycin (Invitrogen), 55 uM 2-b mercaptoethanol (Invitrogen). All cells were maintained at 37°C and 5% CO2 in a humidified incubator. Aicda enhancer E1 was deleted using TALENs approach.