FIGURE 1.

Schematic representations of the replacement fragments and comparison between the amino acid sequences of RSA and ev-1 SU. The amino acid sequences were aligned by using the ClustalW method in the MegAlign program (DNASTAR, Madison, WI, United States). The residues from hr1 to hr2 and the vr3 region were divided into 12 segments (designated s1–s12). ev-1, Subgroup E of avian leukosis virus strain ev-1, GenBank AY013303.1; RSA, Subgroup A of avian leukosis virus strain RSA, GenBank M37980.1. Red and blue letters indicated substitutions of residues in different fragments.