PI3K signalling is the most increased pathway in human cancers. The four isoforms of PI3K are thought to be activated by different redundant mechanisms leading to a common downstream signalling. However, the mutational pattern of PI3K pathway or its level of expression is not sufficient to predict the sensitivity to PI3K inhibitors. By identifying for the first time a phosphopeptide that predict the sensitivity to p110α and/or p110γ inhibitors, the inventors provide insight in how to handle heterogeneity of PI3K expression patterns in tumoral samples for the choice of available PI3K-targetting drugs. Accordingly, the present relates to a phosphopeptide characterized by the amino acid sequence as set forth in SEQ ID NO:1 (PGTPSDHQSQEASQFER) wherein the threonine residue at position 3 is phosphorylated.