The PIK3CA (a.k.a. PI3K-alpha) gene is located on chromosome 3q26.32 and encodes the 110 kDa catalytic subunit of the phosphatidylinositol 3-kinase (PI3K). Amplifications of PIK3CA were found e.g. in cervical, ovarian, endometrial, breast, gastric, and lung cancer. In ovarian cancer as well as cervical cancer cells increased copy numbers were shown to be associated with increased expression of the gene product and PI3K activity. Furthermore, treatment with a PI3K inhibitor leads to decreased proliferation and increased apoptosis. It was concluded that PIK3CA is an important oncogene in these tumors. Likewise in endometrial carcinomas detection of PIK3CA amplification is associated with tumor grade and stage. A significant correlation between PIK3CA amplification and poor survival was found for gastric cancer patients. PIK3CA amplification was also frequently found in non-small cell lung cancer (NSCLC) and was shown to be associated with certain clinicopathologic features. PIK3CA amplification seems to promote tumorigenesis through aberrant activation of the PI3K/Akt signaling pathway. Hence, this pathway might represent an effective therapeutic target in several cancer types.