Chromosome

Each human cell carries 23 pairs of chromosomes, one set from each parent, and each chromosome is a single very long DNA molecule wrapped around proteins. Cancer cells frequently gain or lose whole chromosomes or pieces of them (aneuploidy), swap segments between chromosomes (creating gene fusions), or copy a segment many times over (amplification). Karyotypes and copy-number profiles read these large-scale changes; the Philadelphia chromosome in chronic myeloid leukaemia, which fuses BCR and ABL, was the first such change tied to a specific cancer and to a specific drug, imatinib.