Positive feedback loop of Akt-Skp2 contributes to Tamoxifen resistance, Rapamycin resistance, Bortezomib resistance and Gefitinib resistance. Colorful arrows indicate different signaling pathways. P: phosphorylation; Ub, Ubiquitination. Skp2: S-phase kinase-associated protein 2, NKX3-1: NK3 homeobox 1, OCT-4: Octamer-binding transcription factor, mTORC1: mTOR complex 1, mTORC 2: mTOR complex 2, PI3K: phosphatidylinositol-3-kinase, PDK1: 3-phosphoinositide-dependent protein kinase 1, Akt: protein kinase B, APC/CDH1: anaphase promoting complex/ E-Cadherin, MARCKS: Myristoylated alanine-rich C-kinase substrate, FOXO3: Forkhead box O3, E2F1: E2F transcription factor 1, CREB: cAMP-response element binding protein, EIF4E: eukaryotic initiation factor 4E, HK2: Hexokinase 2, AMPK: AMP-activated protein kinase, MYCN: N-myc proto-oncogene protein.