rTT.C sorted STAT5bER pos clones are monoclonal and have undergone somatic hypermutation A crucial hallmark of memory B cells is that the cells have undergone somatic hypermutation [28] and affinity maturation, the mechanisms via which they become able to develop high affinity antibodies. For this reason we determined the mutation status of TT specific STAT5bER pos memory B cell lines from one donor (Figure 3d and e) . Monoclonality of memory B cell lines that were cultured at clonal density (Table 1 ) could be confirmed by VDJ sequencing per cell line, as only one VDJ sequence per cell line was detected ( Figure 4 ). From this one donor, we obtained three independent immunoglobulin sequences -one IgG and two IgM sequences ( Figure 4 ). Interestingly, one IgG and one IgM had a similar basic pattern of somatic hypermutation, with the IgM sequence containing 6 additional mutations, one of which led to an amino acid change. This suggests that these memory B cells arose from the same clone, and that the IgM clones had undergone additional somatic hypermutation and affinity maturation.