VHH J3 neutralization activity was assessed in the indicated SHIV pseudoviruses, as described in Materials and methods. The derivation of SHIV1157IPD3N4 and SHIV1157IP EL-p is described in detail in Humbert et al. (2008). The SHIV neutralization activity of J3 was assayed in duplicate on TZM-bl cells as described in Materials and methods, the subtype B SHIVs assays were undertaken once, and the subtype C SHIVs were assayed in two independent experiments.