The concept of human B cells expressing exclusively IgM with lack of progression to class-switched variants, despite showing hallmarks of prior activation evolved from an observation that some histogenetic types of human B cell lymphomas had somatically mutated immunoglobulin variable region genes (IGV) indicative of GC derivation, but tended to express IgM and not undergo class switching [83]. The gene discussed is CD40LG; the disease is B-cell non-Hodgkin lymphoma.