Personalized medicine approaches aim to decipher the genetic code of a patient's tumour in order to identify cancers which will behave aggressively and need treatment. However, extensive intra-tumour heterogeneity (ITH) has been discovered in several solid tumour types, complicating the identification of the relevant genetic changes as they may not be present throughout the entire tumour and are likely to be missed by tumour biopsies. ITH is thus a major hurdle for the implementation of personalized cancer treatment approaches.