Many molecules, including XIAP [3], MKP3 [9], PI3K, Akt2 [10,11], PTEN [12], P-glycoprotein [13] and MDR [14], have been reported to be involved in the regulation of apoptosis and in the complicated signaling network that determines the fate of cancer cells, i.e., either "death" or "survival." Though much progress has been made, our recent studies have made efforts to predict the response of cancer cells to chemotherapeutic agents before treatment and to identify the possible alterations that mediate resistance. This evidence concerns the gene PTEN and cancer.