Figure 3. Chain K of antibody b12 (blue) with the CD4 surface receptor protein (red). This is a comparison of the analogous loops on the b12 and CD4 molecules – the portions that insert into the gp120 binding sites. Projecting loops are shown in blue and orange, with analogous W100 and F43 residues shown in aquamarine and yellow spheres, respectively. (pdb code: 1hzh for b12 and 1cdj for CD4)