Extinction of expression of the translocated myc gene in somatic cell hybrids between mouse myeloma and l‐cells

Most murine plasma‐cell tumors show a t(12;15) reciprocal chromosomal translocation which truncates the first exon of one of the myc gene alleles and fuses it to one of the switch regions of the immunoglobulin (Ig) heavy‐chain locus. This results in constitutive activation of the translocated myc gene and the production of smaller‐sized mRNA molecules, which are initiated at new sites in the first myc intron. The normal myc allele is not expressed in these myeloma cells. We have studied the expression of the translocated myc gene in somatic cell hybrids between mouse myeloma and L‐cells. Our previous findings show that Ig gene expression is extinguished in such hybrids. In the present work we found that the hybrids contain the normal and translocated myc genes. In contrast to the myeloma parental cells which express the translocated myc gene, the hybrids are similar to the L‐cells in expressing only the normal myc allele. Our results suggest that the L‐cell, fibroblast‐like phenotype, is dominant in these hybrids, and show that the translocated myc gene is expressed in a tissue‐specific manner in the context of the myeloma cell, and is not expressed when subjected to a fibroblast‐like cellular environment.