Predominant changes in amino acids between P0 and P7, or P24. Single letter sequences are shown for stock (P0), passage 7 (P7) and/or passage 24 (P24) SIV clones, with the position of each change indicated in blue numbers above the P0 sequence, the P0 residue in red, altered residues in green, and the clonal frequency of mutations indicated for passages. The variable (V), conserved (C), or transmembrane (TM) regions in which mutations occur are indicated. SIV 17E-Fred (A) required few mutations to become CD4 dependent. By contrast, SIV 239CEMx174required multiple mutations by P24 (B) to become CD4 dependent, with many appearing by P7 and persisting through P24 (C).