Potential molecular interactions between gp120 of 17-Sens and 17-Res Envs and the CCR5 N-terminus. Homology models of sensitive and resistant gp120 protein sequences were created using a crystal structure of YU2 gp120 bound to CD4 and a sulfated CCR5 N-terminus peptide mimic . Models of 17-Sens (A) and 17-Res (B) are represented as grey ribbons. Amino acids involved in bonding between gp120 and the CCR5 N-terminus are shown as sticks. The Arg440 side chain is shown in green, to illustrate its potential repositioning from the bridging sheet in 17-Res Env. Purple; CCR52-17 N-terminus peptide, green dashed lines; bonds.