 Selectivity for one coreceptor or the other is especially dependent upon the V3 loop, a highly variable and structurally flexible region of gp120 that is composed of approximately 35 amino acids.Tropism can be predicted through the 11/25 method, which looks for basic amino acids at positions 11 and 25 in the V3 loop and suggests the presence of an X4 virus.Coreceptor usage, however, can change throughout the course of infection.