FIG. 1.

Evolution of two SU amino acid positions that govern BNR in the study of TCA FIV in seven infected cats, as observed in this and previous studies (3, 6). The boxes at each sampling time point represent amino acid positions 481 and 557, as indicated under the parental TCA virus used as the inoculum, and are solid when these positions are replaced by BNR-conferring residues. The shaded area includes the samples with virus exhibiting a BNR phenotype. Note that the precise timing of reversion to BNR is known only for group 1 cats, since group 2 cats were first tested 36 months p.i. Cat 311 yielded the first virus-positive cultures at month 10 p.i. The virus in the sample obtained from cat 583 at 15 months p.i. exhibited intermediate neutralization resistance and yielded two viral sequences, in only one of which was the amino acid at position 481 replaced by a BNR-conferring amino acid (3). The complete sequences of all variants are available from the authors by e-mail.