Acquired genomic deletions in sμ and Cμ. DNA from ∼ 100 sorted CD138⁺ cells, obtained from the spleen of NM and L^−/− mice as in Figure 2, was amplified for 20 cycles using J_H4- and Cμ4-specific primers. (A) Separate reamplification for a further 20 cycles was carried out with (nested) primer combination as indicated: J_H4-Cμ4, J_H4-Cμ3, Eμ-Cμ4, and Eμ-Cμ3. After agarose gel electrophoresis, bands were cut out and reamplified or cloned, before sequencing using the Cμ3 primer. (B) Map of the region encompassing J_H4, Eμ, switch (s)μ to Cμ exons 1-4, with examples from 2 different L^−/− mice, with deletions indicated by the superimposed shading. PCR fragments and sequences obtained from a normal mouse showed the wild-type configuration (top). Amplification and sequencing primers are indicated by (see sequences in supplemental Table 1.)