Chromosomal translocations or DNA rearrangements affecting c-myc occur in almost all murine plasmacytoma and human Burkitt's lymphoma tumors and are associated with a high incidence of exon 2 missense mutations and selective expression of the affected allele. Screening nine multiple myeloma cell lines, we identified no exon 2 missense mutations but did identify two lines with single, silent mutations in exon 1 and exon 2, respectively. Each of these informative multiple myeloma cell lines selectively expresses only one c-myc allele despite the apparent absence of chromosomal translocations or DNA rearrangements affecting c-myc.