Southern blot analysis to detect class switch recombination in L1236 cells. (Upper part) After digestion of genomic DNA with BamHI, the organization of the Cγ genes was analyzed in L1236 cells (lane 1) in comparison with the HD-derived cell line Co (lane 2) known to have germline configuration of the Cγ genes.20 The Cγ probe used in the analysis recognizes all Cγ genes and the ψγ gene. As expected, the DNA of Co shows all germline fragments for the IgG subclasses. The two IgH loci located on the two chromosomes 14 show a polymorphism for the pseudo Cγ (ψγ), the Cγ2, and the Cγ4 genes resulting in two fragments for these (indicated as /2), whereas only one fragment appears for Cγ1 and Cγ3 each. This pattern is in accordance with the results described by Linsley et al.21 For L1236 DNA, only one fragment is detectable, indicating that on one allele class switch recombination was targeted to the most 3′ γ gene (ie, Cγ4), whereas on the other allele all Cγ genes were deleted. (Lower part) Map of the human IgH locus. This map does not take into account the different haplotypes for the ψγ, the Cγ2, and the Cγ4 genes and is not to scale. B, BamHI; E, EcoRI.