Evolution of the structure of C2-V3-C3 envelope region. Three-dimensional structures of C2-V3-C3 amino acid sequences from child 1 and 2 were generated by homology modelling using the three-dimensional structure of an unliganded SIV gp120 envelope glycoprotein as template. A) Superimposition of the structures of C2-V3-C3 of child 1 in 1998 (yellow), 2000 (blue) and 2003 (pale red); B) Superimposition of the structures of C2-V3-C3 of child 2 in 1992 (yellow), 1997 (blue) and 2001 (pale red). V3 loop and C2 and C3 stretches are indicated in the figures. C) Three-dimensional structures of C2-V3-C3 from child 1 (CT) in 2003 (blue) and child 2 (SC) in1997 (red). At this time both viruses were X4 and showed resistance to Nabs.