Graphic representation of the 2 breakpoints in case No. 2.

(A) The 3kb 5′Sμ aberrant fragment was amplified by long-distance inverse PCR. Genomic DNA from case No. 2 was digested with SphI, was self-ligated, and long-distance inverse PCR was performed using 4 primers (5MFA, 5MRA, 5MFB, 5MRB) internal to the 5′Sμ region. The 2 breakpoints were generated by a deletion from the Sμ region to just upstream of Cμ exon 1 and a second deletion from Cμ exon 3 to γ 3 sequences, upstream of the 2 membrane exons. Eμ = intronic enhancer, Sμ = switch μ region, Cμ = constant μ exons, M1, M2 = exons for membrane form, Sγ = switch γ region, Cγ3 = constant γ3 exons. (B) Alignment of the breakpoint regions of case No. 2 with germline GenBank sequences. Numbers indicate the nucleotide position at which the breakpoint occurred within the corresponding GenBank sequence. These breakpoint sequences are available from GenBank under accession numbers AF156 531 and AF156 532. HSJHCMU = GenBank nucleotide sequence of the IgH Sμ region, HSIGCMUDE = GenBank nucleotide sequence of the human IgH Cμ and Cδ genes, D78 345 = GenBank nucleotide sequence of the human Ig γ heavy chain.