To provide a nucleic acid encoding a receptor protein kinase, wherein the nucleic acid has tandem duplication in a nucleotide sequence of a juxtamembrane and is useful for diagnosis of leukemia; a polypeptide encoded by the nucleic acid; an antibody capable of specifically binding to a region encoded by the nucleic acid having tandem duplication occurring in a nucleotide sequence of a juxtamembrane; a nucleic acid capable of specifically binding to the nucleic acid having tandem duplication occurring in a nucleotide sequence of a juxtamembrane; a method for detection of the nucleic acid encoding a receptor protein kinase; and a kit therefor. A nucleic acid encoding a receptor protein kinase, wherein the nucleic acid has tandem duplication in a nucleotide sequence of a juxtamembrane; a polypeptide encoded by the nucleic acid; an antibody capable of specifically binding to the portion of the polypeptide; a nucleic acid capable of specifically binding to the nucleic acid; a method for detection of the nucleic acid; and a kit for detection.