The present invention relates to a method comprising inferring activity of a PI3K cellular signaling pathway in a tissue and/or cells and/or a body fluid of a medical subject based at least on expression levels of one or more target gene(s) of the PI3K cellular signaling pathway measured in an extracted sample of the tissue and/or the cells and/or the body fluid of the medical subject. The present invention further relates to an apparatus comprising a digital processor configured to perform such a method, a non-transitory storage medium storing instructions that are executable by a digital processing device to perform such a method, and a computer program comprising program code means for causing a digital processing device to perform such a method.