An extensive RSV F conformational change brings together HRA and HRB, to form one six-helices bundle stable postfusion structure (Figure ). Such conformational change precedes viral / host membrane fusion by inserting extended hydrophobic F fusion peptides into the host cell membranes. To preserve antibody epitopes for vaccine development, the F prefusion-conformation have been stabilized by including S155C and S290C mutations and eliminating p27, transmembrane and cytoplasmic domains . Such prefusion-stabilized F models have been intensively explored not only for antibody-binding studies but also for small drugs inhibiting RSV fusion and infection .