We have identified a rearranged c-myc gene in DNA from tumour tissue from a patient with an aggressive carcinoma of the breast. Analysis of this rearranged gene isolated from a size-fractionated genomic library revealed that a deletion had occurred within exon 3. This deletion, of approximately 5 kb, interrupts the coding region of exon 3, and would result in a truncated myc protein with an altered C-terminus. Sequence analysis of the rearranged c-myc gene and the sequence downstream of c-myc which is involved in the deletion has shown precisely where the breakpoint has occurred in both sequences and reveals a short region of homology which could perhaps permit illegitimate recombination between the two sequences.