The role of the hepatitis C virus (HCV) p7 protein in the virus life cycle is not known. Previous in vitro data indicated that this 63-aa polypeptide is located in the endoplasmic reticulum and has two transmembrane domains (TMDs) connected by a cytoplasmic loop; the amino- and carboxyl-terminal tails are oriented toward the endoplasmic reticulum lumen. Furthermore, recent in vitro studies suggested that HCV p7 could function as a virus-encoded ion channel. It might therefore be a relevant target for future drug development. We studied the role of HCV p7 in vivo. Because HCV does not replicate efficiently in cell culture, we mutagenized p7 of an infectious genotype la cDNA clone and tested RNA transcripts of each mutant for infectivity in chimpanzees by intrahepatic transfection. Appropriate processing of mutant polypeptides was confirmed by studies in transfected mammalian cells. Mutants with deletions of all or part of p7 and a mutant with substitutions of two conserved residues in the cytoplasmic loop were not viable. Thus, p7 is essential for infectivity of HCV. A chimera in which the p7 of the 1 a clone was replaced with p7 from an infectious genotype 2a clone also was not viable. This finding suggests a genotype-specific interaction between p7 and other genomic regions. To define which portions of p7 played the most significant role for this interaction, we tested three chimeras with the 1 a backbone in which only specific domains of p7 had the 2a sequence. A p7 chimera with 2a tails and TMDs and the 1a cytoplasmic loop was not viable. A mutant with 2a tails and cytoplasmic loop and la TMDs also was not viable. However, a p7 chimera with 2a TMDs and cytoplasmic loop and la tails was viable. The transfected chimpanzee became viremic at week 2, and recovered viruses had the chimeric sequence. These data indicate that the amino- and/or carboxyl-terminal intraluminal tails of p7 contain sequences with genotype-specific function.