gB-specific memory B cells in 15 seropositive individuals. Frequencies of gB-binding, IgG-positive memory B cells among all IgG-positive memory B cells ranged from 0.33 to 1.4%, being in the range of frequencies of IgG memory B-lymphocytes against other viral antigens [42] (Fig. S1B ). Among HCMV seronegative individuals, gB-binding memory B cells were detectable but with considerably lower frequency. These cells might bind gB unspecifically or may represent part of the natural antibody repertoire [43] . Sorted gB-binding B cells were activated and immortalized at the clonal level by an in vitro culture system using CpG oligonucleotides and EBV [44] . Among clonal cultures with IgG secretion 40-95% (mean 63%) of cultures showed IgG binding to gB in ELISA, substantiating a high degree of specificity in the cell sorting process (data not shown).