Another example showed that the PTEN loss signature, developed from the IHC of 105 breast tumors, classified 44% of PTEN positive tumors as PTEN negative when the signature was applied to test samples [21], suggesting that PI3K pathway activity is at least partially regulated by other mechanisms, which include gain-of-function mutations in PI3KCA, and overexpression or hyperphosphorylation of PDK1, AKT1, and mTOR. The gene discussed is AKT1; the disease is breast neoplasm.