A method for analysing insertion sites of an exogenous nucleotide sequence in a subject's genome, wherein the method comprises: (a) providing a sample from the subject comprising cell-free double-stranded DNA polynucleotides; (b) blunting the ends of the polynucleotides; (c) ligating an oligonucleotide to both ends of the polynucleotides; (d) amplifying polynucleotides comprising an insertion site; and (e) sequencing the product of step (d).