Functional class switch recombination may occur ‘in vivo’ in Waldenström macroglobulinaemia

Summary Waldenström macroglobulinaemia (WM) malignant cells have been considered incapable of undergoing class switch recombination (CSR). However, we report a WM patient who developed an IgG M‐component 4 years after diagnosis. When the second monoclonal component appeared, reverse transcription‐polymerase chain reaction showed the presence of pre (Cμ) and postswitch (Cγ) clonotypic isotypes; sequencing of these isotypes demonstrated that both corresponded to the single clone amplified at diagnosis, including the same complementarity‐determining region 3 and somatic mutation pattern. This proves that WM cells can undergo a functional in vivo CSR.