Eight r mutants, which are defective in fusion function, were isolated following BURR mutagenesis of a syncytial strain of herpes simplex virus type 2 (HSV 2). Most of the mutant pairs complemented each other and resulted in cell fusion, thus the r mutants could be classified into seven complementation groups. R mutants belonging to different complementation groups recombined effectively with each other, making it possible to construct a linkage map based on fusion function.

Three r mutants, which defect in fusion function, were isolated When r201 was infected with r94 or r12 possessing double mutation sites, the recombination frequencies were approximately equal to the distance between two loci of the double mutant. These results indicate that r201 is or involves a suppressor gene active on the r mutants of this virus.