Longitudinal analysis of chronic hepatitis C virus (HCV) infection has shown that the virus has several adaptive strategies that maintain persistence and infectivity over time. We examined four serum samples from the same chronically infected HCV genotype 4a patient for the presence of IgG antibody-associated virus. RNA was isolated from antibody-associated and antibody-free virions. Subsequent to sequence analysis, 27 aa hypervariable region 1 (HVR1) peptides were used to test the humoral immune escape. We demonstrated that differential peptide binding of Fab was associated with a single amino acid change. We provide direct evidence of natural humoral immune escape by HCV within HVR1.Hepatitis C virus (HCV) infects 2-3 % of the world population and is a leading cause of liver disease (Freeman et al., 2001;Zhou et al., 2014). Early in infection the host immune system responds by producing neutralizing antibodies (Terilli & Cox, 2013). Although HCV infection stimulates a strong immune response, it is generally insufficient to eradicate infection, as 50-80 % of the infected individuals develop chronic liver disease (Deng et al., 2013;Freeman et al., 2001; Inchausp e et al., 2008;Kenny-Walsh, 1999). The high rate of viral persistence is thought to be a result of a complex interplay between viral diversity and suboptimal immunity. Viruses with hypervariable genomic regions evade host humoral immune response by several mechanisms (Brown et al., 2005;Quaranta et al., 2012;Thimme et al., 2006). The best understood mechanism for viral immune escape is single-point mutation which results in non-synonymous changes within the immunodominant viral envelope glycoprotein and NS3 (Cox et al., 2005; Ray et al., 2005;Thimme et al., 2006Thimme et al., , 2012. Multiple linear epitopes within the 27 aa hypervariable region 1 (HVR1), in the N terminus of the E2 envelope protein, have been identified as the principle target of neutralizing antibodies (Ball et al., 2014;Fafi-Kremer et al., 2012;Tarr et al., 2015). Antibodies specific for epitopes within HVR1 have been reported to inhibit the binding of the E2 glycoprotein to cells and to block HCV infectivity in vitro and in vivo (Farci et al., 1996;Habersetzer et al., 1998; Owsianka et al., 2001). However, HCV pseuodparticle and cell-culturederived HCV experiments have shown poor cross-neutralization potential of isolate-specific neutralizing antibody response to HVR1 (Brown et al., 2005;Cashman et al., 2014;Larrubia et al., 2014). Cytotoxic T-lymphocytes drive evolution of the HVR1, which can lead to the emergence of escape variants (Cox et al., 2005; Ray et al., 2005). However, there is an absence of direct in vivo evidence of humoral immune escape by host-derived antibodies and viral glycoproteins (Chung et al., 2013).Previous research from our group has observed, over a near 10 year period, the emergence, dominance and disappearance of distinct but related lineages (L1 and L2) in a treatment-naive patient chronically infected with HCV genotype 4a (Palmer et al., 2014). L1 domi...