S-RNase is a style-specific ribonuclease which is associated with gametophytic self-incompatibility. An expression vector of a fusion protein of Pyrus pyrifolia (Japanese pear) S3-RNase with glutathione-S-transferase (GST) was constructed and transformed into E. coli. Using this system, the fusion protein, GST-S3-RNase, was expressed as an active form and can be used for screening pollen S-gene product(s).