The PIK3CA gene encodes for the catalytic subunit (p110) of phosphatidylinositol-3-kinase (PI3K). Phosphatidylinositol-3-kinases participate in the regulation of signalling pathways that are responsible for processes involved in cell division, survival, adhesion, and movement. Improper functioning of these processes may lead to tumour formation. Erroneous functioning may be caused, among others, by an activating mutation in the PIK3CA gene, which has been detected in many types of tumours (colon, gastric, and breast cancer, and certain brain tumours). The PIK3CA mutational status plays an important role in targeted tumour therapy since the presence of the mutation confers resistance to EGFR inhibitors.