Transfection efficiency was determined with a siGLO RISC-Free transfection control condition 48 hr after transfection by fluorescence microscopy, which was confirmed to be >90% for all experiments reported. siRNA reagents: siGLO RISC-free (Dharmacon, cat# D-001600–01), siGenome non-targeting control 2 (Dharmacon, cat# D-001210–02), siGenome SERINC1 (Dharmacon, cat# M-010725–00), siGenome ZNF460 (Dharmacon, cat# M-032012–01), siGenome VAPA (Dharmacon, cat# M-021382–01), siGenome CNOT6L (Dharmacon, cat# M-016411–01), and siGenome PTEN (Dharmacon, cat# M-003023–02). 3’UTR plasmids: pcDNA-SERINC1-3’UTR, pcDNA-VAPA-3’UTR1, pcDNA-VAPA-3’UTR2, pcDNA-CNOT6L-3’UTR1, pcDNA-CNOT6L-3’UTR2, pCMV-MCS-PTEN-3’UTR (shared by Dr. Pier Paolo Pandolfi, Beth Israel Deaconess Medical Center).