Clonotypic switch (CS-PCR) amplification in MM patients. (A) A diagram of the rearranged immunoglobulin heavy chain locus: V indicates variable; D, diversity; JH, junction segments; Eμ, intronic enhancer; Iμ, I exon; Sμ, switch region before class switching; Sμ/Sx, hybrid switch region generated after class switching; C, downstream constant region; and x, α (IgA) or γ (IgG) isotope regions. (B) CS-PCR-A and CS-PCR-B, with patient-specific CDR2 and CSR-specific Sx1 or CxB primers to enable selective amplification of clonotypic switch products. For some cases, CS-PCR-A products were used in a nested CS-PCR-B reaction, called CS-PCR-AB, to increase sensitivity. (C) Sensitivity assay for the CS-PCR-A reaction using LP1 cells diluted into normal blood. Cell concentration is shown above the panel; neg indicates no LP1 cells added.