In thesetting of breast cancer, this represents the most commonly deregulated signalingpathway, with alterations that can be summarized as follows: i) overexpression ofPI3K-activating RTKs; ii) inactivating events of negative PI3K pathway regulators (thatis, phosphatase and tensin homologue (PTEN) and inositol polyphosphate 4-phosphatasetype II); and iii) activating events of PI3K pathway components and/or positiveregulators. The gene discussed is PIK3CA; the disease is breast cancer.