carrying a Luc reporter showed that HK20 exhibits reasonably broad neutralization consistent with the results from the HOS cell assay, and similar to Fab HK20 in the TZM-bl cell assay. Whereas the TZM-bl assay is sensitive for certain antibodies such as CD4bs mAbs, some mAbs such as 2G12 to other epitopes are less sensitive to neutralization in this assay [72] , perhaps owing to the unnaturally high level of CCR5 expression on the TZM-bl cells [60] . In this respect, low coreceptor expression levels have been associated with delayed fusion kinetics and hence enhanced virus sensitivity to HR-1 binding inhibitors, such as T-20 [73] . Therefore, despite the advantages of high throughput with the TZM-bl assay, alternative neutralization assays might have closer relevance to the in vivo situation.