Rapamycin and its target TOR are central elements in a signaling pathway that regulates cell growth, survival and proliferation. TOR plays a pivotal role in the biology of malignant cells and TOR inhibitors, including rapamycin, have a potent anticancer effect in many types of tumours. Clinical trials of rapamycin derivatives in cancer patients are currently underway and hold great promise. My laboratory has recently purified a protein complex comprising a target of TOR. Our results indicate that the new complex, termed the RPAP3 complex, participates in the biogenesis of RNA polymerase II, a key enzyme in cellular function. In this application, we propose to characterize this novel protein complex and analyze its role and mechanism of action in TOR signaling. A complete understanding of the TOR pathway is essential for its optimal use as an anti-tumour target.