A series of mouse myeloma mutants, derived from a cell line of the murine MPC-11 tumor (y2b, kappa), resemble human heavy-chain disease in their loss of an internal domain (exon). In these mutants, most of the -y2b CH, exon was present in the nuclear RNA but was removed during splicing to form the mature cytoplasmic RNA. Amino acid sequence studies of one mutant (10.1) are consistent with the loss of the complete CH, domain. A second mutant cell line (I17) derived from 10.1 and containing the same CH, alteration was shown by Si nuclease protection experiments to have an additional mRNA deletion spanning the CH_-CH3 domain boundary. This second deletion was shown to result from a genomic alteration that provided a marker for the isolation of the expressed H-chain allele. To determine the basis of the CH, splicing defect, the 117 genomeexpressed y2b constant region DNA was cloned. Sequence studies showed a deletion of 99 nucleotides around the 3' end of the CH, domain, which removed the splice site and flanking DNA, apparently causing the aberrant splicing of the RNA transcript. The sequence deleted in the mutant is flanked by short repeats of the octameric sequence CCAGCCAG in the wild-type gene. In the mutant, one copy of the repeat, in addition to the sequences between the repeats, has been lost.Many eucaryotic genes have been found to contain intervening sequences (IVSs) which interrupt coding regions. The IVSs are transcribed with the coding sequences and are removed by splicing to generate mature functional RNA. Splicing has been shown to be important for regulation of gene expression in many cases (12). The study of splicing mutants will be important in elucidating the mechanism of splicing and how it is controlled. Some mutants with DNA alterations consistent with errors in splicing have recently been described (16,17,22,23,31,37,38). In this communication, we describe experiments showing that a series of immunoglobulin heavy-chain deletion mutants exhibit altered processing of the nuclear precursor RNA. These mouse myeloma mutants share some features with human heavy-chain disease.Several human heavy-chain disease proteins have been shown to have internal deletions beginning or ending near the hinge (20,21). The molecular explanation for these altered proteins remains obscure since the DNA and parental cell from which they arose are not available for study; altered RNA splicing may explain the origin of some. The myeloma cell culture system is well suited for elucidating the mechanisms by which mutants in immunoglobulin production arise and for generating mutants in gene expression. The mutation frequency in the immunoglobulin genes is high (1, 10); mutants can be readily identified and isolated (9). Parents and progeny can be compared directly to localize significant DNA alterations.We have previously described the mRNA and proteins produced in a series of myeloma mutants derived from the MPC-11 cell line 45.6. Mutant 10.1 arose spontaneously from 45.6 and synthesizes a y2b heavy chain with a deletion of...