The DCPC 21 plasmocytoma lacks any of the MPC-associated chromosomal translocations. However, the c-myc gene has been transposed to the IgH locus on chromorome 12 by an Ig switch-region-mediated recombination mechanism. DNA sequencing analysis, further, revealed that this recombination is consistent with an insertion of the IgH enhancer (Eμ)-Sμ sequences, 2341 bp in length, into the c-myc 5′-flanking region, resulting in 5′: c-myc 5′-flanking-Eμ-Sμ-c-myc 5′-flanking-c-myc exon-1 : 3′ segment. In situ molecular hybridization of DCPC 2I metaphase chromosome spreads using a Pvt-1 probe demonstrated that Pvt-1 has also moved to the FI sub-band region of chromosome I2 where the IgH genes are located. These results indicate that the c-myc gene has been inserted into the IgH locus together with the Pvt-I, regardless of whether plasmocytoma has cytogenetically identifiable translocations. The possible interaction between c-myc activation and Pvt-I in the development of MPCs is discussed.