B lymphocytes may switch from producing an immunoglobulin heavy chain of the μ class to that of the γ, ε or α class1-4. To maintain the specificity, the new heavy chain must keep the original variable (V) region; this is achieved by deleting DNA sequences so that the V (consisting of joined V H, diversity (D H) and joining (J H) gene segments) and C (constant) gene segments coding for the new heavy chain are brought into close proximity (reviewed in ref. 5; we do not consider here the μ-δ situation). There are, in principle, three types of chromosomal rearrangements that yield a deletion: (1) rearrangement within a chromatid; (2) unequal sister chromatid exchange (as suggested by Obata et al. 6); and (3) unequal recombination between chromosomal homologues. We have analysed the arrangement of C μ DNA in clones of the pre-B-cell line 18-81 that switches in vitro from μ to γ2b (ref.7). The clones examined produce either μ, γ2b or no immunoglobulin chain. We report here that all the γ2b clones had lost at least one copy of C μ and no clones contained three copies of C μ. These findings formally exclude both unequal sister chromatid exchange and recombination between homologues as mechanisms for creating a gene encoding the γ2b chain. © 1985 Nature Publishing Group.