Discloses a method for predicting whether a tumour will respond to a therapeutic agent that targets a protein in the PI3K (phosphatidylinositol-3-kinase) pathway this method comprising determining whether a tumour comprises a variation in a: (i) PIK3R1 (phosphatidylinositol 3-kinase regulatory subunit alpha 1) polynucleotide (ii) or PIK3R1 polypeptide that results in activation of a PIK3CA (phosphoinositide 3-kinase catalytic alpha polypeptide polypeptide). Further discloses the use of antagonist of PI3K signalling for the manufacture of a medicament for treating a tumour in a mammal, wherein the tumour has been determined to comprise an amino acid variation in a PIK3R1 polypeptide and the amino acid variation results in activation of a PIK3CA polypeptide.
