PTEN and breast cancer: 25). PTEN is a tumour suppressor gene involved in translation, cell cycle and apoptotic processes. PTEN suppresses apoptosis and increases cell survival by negatively regulating the AKT kinase pathway (Ref. 75). Moreover, PTEN is involved in DNA repair processes and is essential for BC signalling pathways. The downregulation of PTEN leads to the development of malignant mammary stem/progenitor cells through increased signalling within the AKT/GSK-3β/Wnt/β-catenin pathway; moreover, the loss of PTEN results in resistance to trastuzumab therapy and poor OS (Ref. 76).