Fromalibrary ofmousesperm DNA,wehave isolated twooverlapping clones which contain theCc, gene. One ofthese clones also contains theC,6 gene. TheC gene isseparated fromtheC membrane exons byapproximately 2kilobases (kb) ofDNA.AeCc, genewasidentified by(a)hybridization to poly(A) RNAprepared fromtheIgD-producing ratplasma cell tumorIR731, and(b) homology ofatranslated nucleotide se- quence totheaminoacid sequence ofthehuman 8chain. TheC genespans 8kbofDNAinthegermline. Plasmid subclones of theCc, genewereused asprobes inSouthern andRNAblot ex- periments. RNAblot analysis ofcytoplasmic poly(A)+RNA from 1R731 andapiA+8 B-cell hybridoma revealed 1.6- and2.7-kb 8 mRNAspecies with different 3'ends, which presumably encode thesecreted andmembrane-bound forms, respectively, ofthe8 chain. Southern blot analysis ofDNAfrom two,u+8+ lymphomas revealed that theC5g gene isinthegerm-line configuration ineach case. Restriction mapanalysis ofC andCc, genomic clones iso- lated from alibrary ofnormal IA+ 8B-cell DNAalso gave noevi- dence forDNArearrangement intheregion between theC,and Cog genes. Taken together, these data suggest that IgDexpression inpI+B+ Bcells does notinvolve aVH-to-Cs DNA switch rear- rangement. Wepropose that simultaneous expression ofCAand Ccwith asingle V geneismediated bytwoalternative routes of RNAprocessing o?aprimary nuclear transcript which contains theVH,C ,andCc, genes. Incontrast, analogous experiments with myeloma TR731 DNArevealed that theC,, genehasbeen deleted from themyeloma DNAandthat theCog gene hasundergone DNA rearrangement, presumably including aswitch recombination of theVHgenefromtheC totheCc, gene. These results indicate that twoalternative meclanisms maybeused intheexpression of IgDmolecules-RNA splicing inBcells andDNArearrangement inplasma cells.