Chromosomal abnormalities in tumours were recognized at the end of the last century but their significance has only recently become clear. Distinct translocations in leukaemias and in solid tumours lead to the activation of proto-oncogene products or, more commonly, creation of tumour-specific fusion proteins. The proteins in both categories are often transcription factors and thus disruption of transcriptional control plays a major role in the aetiology of cancer. Fusion proteins formed after chromosomal translocations are common in a range of tumour types; these are unique tumour antigens and are therefore potential targets for therapy design.