Abstract: A method of characterising a DNA sample obtained from a tumour, the method including the steps of: (a) obtaining a tumour copy number profile for the sample, (b) quantifying a set of copy number features of the copy number profile, and (c) determining exposure to one or more signatures of chromosomal instability based on the quantified features. A copy number feature is a metric that characterises a copy number event in a copy number profile. The set of features does not comprise the absolute copy number of segments in the copy number profile. The signatures of chromosomal instability have been obtained by quantifying the set of copy number features in a plurality of tumour samples, and identifying one or more mutational signatures likely to result in the copy number profiles of the plurality of tumour samples. Methods of characterising types of chromosomal instability present in samples, providing a prognosis, identifying a drug target, or identifying a therapy for a subject are also described.