FIG. 1.

Schematic diagram of the IBV S gene. The 5′ end of the S gene overlaps the 3′ end of the replicase gene. The four domains of the S protein, the position of the S1/S2 cleavage point, and the positions of the PacI and BspHI restriction sites, gene 3 TAS, and the start of gene 3 are shown. The numbers refer to the positions of the amino acid differences resulting from nonsynonymous substitutions following exchange of the two S gene sequences.