Recombinant virus phenotypic entry assay.Sensitivity of deep sequencing for detecting minor CXCR4-using variants. We determined the sensitivity threshold of NGS by calculating the error rate, either globally or for each position of V3. The mean frequency of V3 variant artifacts determined after analysis of 20 virus clones with the Illumina was 0.078 [exact Poisson 99 confidence interval (CI), 0.