PURPOSE: To obtain a gene coding the ribosomal RNA of Valencia orange and usable as a base for designing a probe or primer suitable for the judgment of the genetic identity of citrus plant varieties, to obtain a probe or primer to be used for the judgment of the genetic identity of citrus plant varieties and to provide a method for judging the genetic identity of citrus plant varieties using the probe or primer.

CONSTITUTION: This invention relates to a gene coding the ribosomal RNA of Valencia orange containing a specific base sequence; a probe for detecting the restriction fragment length polymorphism of the gene of a citric plant containing a part or total of the base sequence of the spacer region of a gene coding the ribosomal RNA of a citric plant; a gene amplification primer capable of amplifying a part or total of the base sequence of the spacer region of a gene coding the ribosomal RNA of a citric plant; a gene amplification primer containing a specific base sequence; and a method for detecting the restriction fragment length polymorphism of a gene of a citrus plant by southern blotting process using the probe for detecting the restriction fragment length polymorphism.