AKT1 and neoplasm: In HER2+ tumours, the MAPK/ERK pathway is activated through ERBB2, but PI3K/AKT pathway activation is additionally required, for example, via ERBB3 (HER3) upregulation (Fig. 8d), PIK3CA mutations or PTEN loss44, 45, 46, 47, 48, 49.