We have been investigating the involvement of the Ras/Raf/MEK/ERK, PI3K/PTEN/Akt/GSK-3 and TP53 signaling pathways in breast, pancreatic, prostate, hepatocellular, and hematopoietic chemotherapeutic drug and radiation resistance. We have determined that altered expression of these pathways can increase resistance to both chemotherapeutic drugs and ionizing radiation. We are during the involvement of these pathways in cancer stem cells which we have been isolating from these different tumor types with the goal of being able to specifically target these pathways in the cancer stem cells. The PI3K/Akt pathway is also important in resistance to Tamoxifen therapy in breast cancer. This is important as the phosphatidylinositol-3 kinase (PI3K) gene(PIK3CA) is frequently mutated in breast cancer which results in Akt activation. This suggests that targeting PI3K may prove effective in therapy of drug, radiation and hormonal resistant breast cancer. We have determined that expression of mutated forms of the PTEN phosphatase and GSK-3beta kinase will increase the resistance of breast cancer cells to chemotherapeutic drugs and ionizing radiation. Thus these mutants act as dominant negative mutants to suppress the activity of the wild type protein. This novel discovery is important as mutations at the PTEN gene can occur in a heterozygous state and could act to silently increase chemo- and radio- resistance. We have determined that the lipid phosphatase as well as protein phosphatase activities of PTEN are important in determining resistance to chemotherapeutic drugs and ionizing radiation. This is important as it indicates that protein as well as lipid substrates are important in determining breast cancer chemo and radio sensitivity/resistance. Furthermore, we have determined that expression of dominant negative or wild-type TP53 will alter the chemo and radio-sensitivity of breast, pancreatic, prostate, hepatocellular and hematopoietic cells. Further analysis should provide clues into which genes and signaling pathways become activated or inactivated during the conversion of the breast, pancreatic, prostate and hematopoietic cancer cells into therapy resistant variants which may lead to more effective targeting. This is providing substantial rationale for the targeting of specific pathways in breast, pancreatic, prostate, hepatocellular and hematopoietic cancer.