the present invention relates to a linear dna vector and to a method for identifying chromosomal regions having a physical proximity within a living cell , and / or for locating in chromosomes of a living cell a dna double strand break having a physical proximity with a known non - repetitive dna sequence . the linear dna vector has a first end which comprises a nucleotide sequence capable of homologous recombination to a first region of a cell chromosome which comprises a known non - repetitive dna sequence . the linear dna vector also has a second end which comprises a nucleotide sequence non - homologous to the chromosomes of the cell and being capable of illegitimate integration with a second region of a chromosome in physical proximity to the first chromosomal region .