FIG. 1.

(A) Schematic representations of the ALV-based RCASBP replication-competent retroviral vector and the major domains of the envelope glycoproteins. The five regions of amino acid sequence variation (vr1, vr2, hr1, hr2, and vr3) identified by comparing the sequences of the surface glycoproteins (SU) of ALV(A) to ALV(E) are also shown. (B) Comparison of the amino acid sequences of the two major SU variable domains, hr1 and hr2, of representative ALV(A) to ALV(E). The sequences were aligned with the ClustalW multiple-sequence alignment program of MacVector 6.5. Amino acids identical to SR-A are denoted by dots, and gaps in the alignment are denoted by dashes. (C) Comparison of the amino acid sequences of the quail and chicken Tva receptors homologous to the LDLR cysteine-rich ligand binding domain. Only the amino acid differences are shown for chicken Tva. The six cysteines are in bold, and the disulfide bonds are indicated by brackets.