ES cells from Ighg1tm1(IRES-cre)Cgn heterozygous mice were targeted at the ROSA26 locus with a conditional transgene encoding a constitutively active form of the STAT-3 protein proceded by a floxed STOP cassette and followed by an IRES-EGFP cassette. Upon SRBC immunization, STAT-3c expression is detected in the majority of germinal centers and IgG1 memory B cells by EGFP expression. (J:109583)