Previously we have published that ectopic expression of active STAT5 mutants in human primary B cells results in a block in B cell differentiation and that these cells show enhanced survival and expansion [16] . We extended these findings by showing that constitutive activation of STAT5 in B cells led to loss of antibody surface expression when cultures were maintained for more than 6 weeks in the presence of IL-2 and IL-4 [17] . We then investigated whether we could exploit the immortalizing capacity of active STAT5 mutants in order to obtain human monoclonal B cell lines, which secrete antigen specific antibodies. For this, peripheral blood CD27 pos memory B cells were cultured in the presence of irradiated CD40-Ligand expressing L cells (CD40L-L cells) and interleukin (IL)-21 prior to retroviral transduction with constitutively activated (CA) STAT5b (CA-STAT5b). Pre-stimulation with IL-21 induced strong proliferation, which resulted in transduction efficiencies of 40 to 80% [15, 18] . Following transduction, in-active CA-STAT5b is expressed as a chimera with the hormone-binding portion of the Estrogen Receptor (ER) (CA-STAT5bER) [16] , thereby STAT5b is expressed in the cytoplasm complexed by heat-shock proteins. Incubation with tamoxifen (4-HT) leads to dissociation of the heat shock proteins from constitutively active STAT5bER, which results in receptor dimerization and subsequent translocation to the nucleus where it initiates STAT5 specific transcription [16, 19] . First we studied the effect of STAT5 transduced cells cultured with IL-2, IL-4 and tamoxifen or with IL-21 and tamoxifen. IL-2 and IL-4 induce proliferation of human primary B cells similar to IL-21 but IL-21 has been shown to also induce strong differentiation of B cells [20, 21, 22, 23, 24, 25] . The secreted immunoglobulin levels in the supernatant of the CA-STAT5bER B cells were measured over time. Around 3 to 4 weeks after transduction the level of secreted IgG started to decrease for the tamoxifen/IL-2 plus IL-4 and tamoxifen/IL-21 cultured cells (Figure 1a ). We also determined the long-term effect of constitutively active STAT5 expression on IgM secretion. IgM expressing peripheral blood B cells were transduced with CA-STAT5bER and cultured with tamoxifen, IL-2 and IL-4. Similar to the IgG secretion levels, IgM secretion disappeared in CA-STAT5bER B cells after prolonged culture (Figure 1b) .