Clonal diversification of PCT by isotype switching in situ as shown by serial tissue sections of a PCT that arose in a C.IL6/iMyc^Cα mouse. Ig heavy-chain–producing tumor cells or κ light-chain–producing tumor cells (top right) are immunolabeled in brown, using isotype-specific antibodies (original magnification, ×40). The bottom right panel presents a molecular scheme on the putative derivation of the γ2a⁺, γ2b⁺, and α⁺ subclones from the parental γ1⁺ clone. The possibility that CSR to Cϵ also occurred was not examined, but is an extremely rare event in other studies of PCT.