In eukaryotes, the PIKK (phosphatidyl inositol 3' kinase-related kinases) co-ordinate the cellular response to DNA damage and play critical roles in the control of cell growth, gene expression and V(D)J recombination. Dysfunction of the PIKK leads to clinical manifestations ranging from immunodeficiency to cancer. The PIKKs are a small but essential group of kinases present in all eukaryotes surveyed to date. PIKK members are high molecular mass proteins and of the six members found in mammals, five (ATM, ATR, mTOR, DNA-PK and SMG-1) function as protein serine/threonine kinases.

The molecular target of rapamycin (mTOR), first described as the kinase potently inhibited by the immunosuppressant and antiproliferative agent rapamycin (RAP), is a central modulator of cell growth and a prime strategic target for anti-cancer therapeutic development. mTOR plays a critical role in transducing proliferative signals mediated through the phosphatidylinositol 3 kinase (PI3K)/protein kinase B (Akt) signaling pathway. mTOR does this by activating downstream protein kinases that are required for both ribosomal biosynthesis and translation of key mRNAs of proteins required for promoting the G(1) to S phase transition.