It is important to emphasize that while either sirolimus or temsirolimus can induce prolonged proliferative arrest based on the absence of PCNA staining, we did not detect cleaved caspase 3 in treated tumor cells (Figs. 4a, 5a, and S13), indicating that neither agent is cytotoxic as a single agent. The gene discussed is PCNA; the disease is neoplasm.