Protocol GS-US-196-0140: A Phase 2b, Randomized, Double-Blind, Placebo-Controlled Trial Evaluating 16 and 24 Weeks of a Four-Drug Regimen and 24 Weeks of a Three-Drug Regimen of GS-9451, Peginterferon Alfa 2a (PEG, Pegasys) and Ribavirin (RBV, Copegus) Wi