As less than 1% of PCM involve IgM producing cells, successful class switching has occurred in the remainder and this would seem to suggest that the mutagenic “hit”, which has resulted in the growth of a monoclonal population of plasma cells has occurred at a later, rather than earlier, stage of B cell ontogeny, and the somatic hypermutation and Ig class switching stages are plausible candidates. The gene discussed is CD40LG; the disease is paracoccidioidomycosis.