The cellular oncogene c-mos is rearranged in a mouse myeloma and the tumour mRNA contains transcripts hybridizing with a v-mos probe. The rearranged gene (rc-mos) was cloned in lambda phage and shown to transform mouse fibroblasts in transfection assays, rc-mos differs from its progenitor, c-mos, only at the 5' end of the gene, where c-mos sequences have been substituted by a novel cellular DNA fragment. This fragment contains a 159-base pair (bp) insertion sequence (IS)-like element localized immediately 5' to the junction with c-mos. This is the first demonstration in a non-virally-induced tumour of activation of a cellular oncogene by a mechanism possibly involving DNA transposition.