The STAT5bER retroviral construct has been described previously [16] . In brief, cDNA encoding STAT5bER was expressed in the Lazarus (LZRS) vector upstream to a internal ribosomal entry site (IRES) and a marker gene (a signaling- Indicated is the total number of clones isolated and at which cell density they were seeded. One 96 well plate was used for each condition (1, 5 or 10 cell per well). Wells contained 2500 CD40L-expressing L cells, IL-2 and IL-4. J to K of the medium was replaced twice a week with fresh cytokines and 5000 L cells. The horizontal bar of each clone represents the sequenced region. The FR1 to CDR3 region was sequenced for clone 5D1 and for clones 10E1 and 10B7 region FR2 to CDR3 were sequenced. We were unable to obtain reliable sequence data of the FR1-CDR1 region of clone 1E10. Cell culture B cells (2610 5 ) were co-cultured on c-irradiated (100 Gy) mouse L cell fibroblasts stably expressing CD40L (CD40L-L cells, 10 5 well) and recombinant human IL-2, IL-4 or IL-21. Cells were routinely tested by PCR for the presence of mycoplasma and EBV (data not shown).