A gp41 MPER-specific llama VHH requires a hydrophobic CDR3 determinant for neutralization but not for antigen recognition by Weissenhorn W, Lutje Hulsik D, Hock M, Liu YY, Strokappe NM, Khattabi ME, Langedijk JP, McCoy LE, Forsman-Quigley A, Aasa-Chapman MM, Weiss RA, Verrips TC, Rutten L