LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II Design Open-label W4 W6 W8 SOF/VEL + GS-9857 > 18 years Genotype 1 or 3 Treatment- naïve or-experienced No cirrhosis or compensated cirrhosis No HBV or HIV co-infection G E N O T Y P E 1 Naïve, no cirrhosis N = 30 SOF/VEL + GS-9857 Naïve, cirrhosis N = 15 SOF/VEL + GS-9857 PEG-IFN + RBV experienced, cirrhosis SOF/VEL + GS-9857 N = 17 PI-experienced, ± cirrhosis N = 28 SOF/VEL + GS-9857 DAA-experienced , ± cirrhosis N = 30 SOF/VEL + GS-9857 SOF/VEL: 400/100 mg FDC qd; GS-9857: 100 mg qd GT3 Naïve, cirrhosis N = 18 SOF/VEL + GS-9857 PR- experienced, cirrhosis N = 19 SOF/VEL + GS-9857 DAA- experienced, ± cirrhosis N = 4 SOF/VEL + GS-9857 Objective SVR12 (HCV RNA < 15 IU/ml), by ITT LEPTON Gane EJ, Gastroenterology 2016; 151:448-456

LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II Baseline characteristics and outcome in genotype 1 with 4 or 6 weeks of therapy 4 weeks 6 weeks Naïve No cirrhosis N = 15 Cirrhosis Prior DAA failure * ± cirrhosis N = 30 Age, years, mean 54 50 59 55 Female 40% 53% 27% 20% White 80% 93% 90% HCV RNA, log10 IU/ml, mean 6.3 6.0 IL28B CC 33% 100% 17% Genotype 1a 73% 77% SVR12 87% 67% Cirrhosis: 60% No cirrhosis: 68% All failures were relapses * Danoprevir + mericitabine, N = 14 ; danoprevir + mericitabine + ritonavir + RBV, N = 6 ; DCV + VX135, N = 3, SOF/LDV + RBV, N = 1 ; faldaprevir + deleobuvir + RBV, N = 4 ; TVR + lomibuvir ± RBV, N = 2 LEPTON Gane EJ, Gastroenterology 2016; 151:448-456

LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II Baseline characteristics, disposition and outcome in genotype 1 with 8 weeks of therapy or in genotype 3 8 weeks 6 weeks Genotype 1 PEG-IFN + RBV experienced Cirrhosis N = 17 Genotype 1 PI-experienced ± cirrhosis N = 28 Genotype 3 Naïve N = 18 PEG-IFN + RBV experienced N = 19 DAA experienced ± cirrhosis N = 4 Age, years, mean 58 57 52 55 56 Female 18% 32% 44% 21% White 94% 86% 67% 95% 75% HCV RNA, log10 IU/ml, mean 6.3 6.1 6.9 IL28B CC 35% 14% 56% 42% 100% 39% 50% Discontinued therapy SVR12 Cause of failure Relapse Lost to follow-up 89% 3 83% 2 1 LEPTON Gane EJ, Gastroenterology 2016; 151:448-456

LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II SVR12 according to baseline NS5A RASs in genotype 1 with 6 weeks of therapy Treatment naïve (N = 30) Baseline RASs = 16  SVR12 in 100% No RASs = 14  SVR12 in 79% Prior DAA failure (N = 29) Baseline RASs = 13  SVR12 in 77% No RASs = 16  SVR12 in 63% SVR12 according to baseline RASs in genotype 1 with 8 weeks of therapy or in genotype 3 No RASs = 46  SVR12 in 97.8% NS5A RASs = 23  SVR12 in 87% NS3 RASs = 26  SVR12 in 85% LEPTON Gane EJ, Gastroenterology 2016; 151:448-456

LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II SVR12 according to prior failure Failure to PI-based therapy, N = 28 Baseline NS3 RASs in 15/28 (54%)  SVR12 in 13/15 (87%) Failure to NS5A inhibitor (N = 7) Baseline NS3 RASs in 6/7 (86%)  SVR12 in 5/6 (83%) Relapse, N = 28 No emergence of RAS, N = 26 No RAS at baseline and failure, N = 15 Same RASs at baseline and failure, N = 6 Baseline RASs but no RAS at failure, N = 5 Emergence of RASs, N = 2 Treatment-naïve, 6 weeks of treatment: NS3 RAS V55A emerged at 2% of the viral population at the time of relapse PI-experienced, 8 weeks of treatment: NS5A RAS Y93H emerged at 2% of the viral population at the time of relapse in addition to the preexisting NS3 RAS R155K LEPTON Gane EJ, Gastroenterology 2016; 151:448-456

LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II Adverse events and laboratory abnormalities, % 4 or 6 weeks 8 weeks 6 weeks Genotype 1 Naïve or DAA-experienced N = 75 Genotype 1 PEG-IFN + RBV experienced Cirrhosis N = 17 PI-experienced ± cirrhosis N = 28 Genotype 3 Naïve N = 18 PEG-IFN + RBV experienced N = 19 DAA experienced ± cirrhosis N = 4 Any adverse event 77% 88% 79% 83% 75% Grade 3 AE Serious AE 4% (n = 1) * 11% (n = 2) * Discontinuation for AE AE in ≥ 10% of patients Nausea 24% 6% 11% 17% 37% 25% Headache 18% 33% 21% Fatigue 16% Diarrhea 9% 22% 50% Grade 3-4 laboratory abnormalities 12% 7% * Atrial fibrillation, hepatocellular carcinoma and bladder cancer, all unrelated to study drug LEPTON Gane EJ, Gastroenterology 2016; 151:448-456

LEPTON Study: SOF/VEL + GS-9857 genotype 1 or 3 Phase II Summary SOF/VEL + GS-9857 for 6 weeks was highly effective in treatment- naïve genotype 1 patients without cirrhosis Shortening treatment to 4 weeks was associated with very low SVR12 SOF/VEL + GS-9857 for 8 weeks resulted in high SVR12 rates in difficult-to-cure, treatment-experienced patients 100% in cirrhotic PEG-IFN + RBV-experienced genotype 1 and genotype 3 89% in PI-experienced genotype 1 Baseline NS3 RASs had limited impact on SVR rates among PI- experienced patients treated with SOF/VEL + GS-9857 for 8 weeks SVR12 of 87% if RASs at baseline vs 92% if no RAS The combination was safe and well tolerated LEPTON Gane EJ, Gastroenterology 2016; 151:448-456