Disclosed are: a novel mutant gene involved in chronic myeloproliferative disorder (CMPD), particularly a novel mutant gene involved in the development of CMPD and carried by a patient who develops CMPD in spite of the fact that he is negative for JAK2V617F; and a method for evaluating CMPD. The susceptibility to CMPD can be evaluated by detecting the following mutations (a) to (c) in JAK2 gene or EPOR gene in a human-derived biological sample: (a) the mutation of G located at position-2116 to A in the nucleotide sequence depicted in SEQ ID NO:1; (b) the deletion of nucleotide residues lying between position-2121 to position-2126 in the nucleotide sequence depicted in SEQ ID NO:1; and (c) the mutation of C located at position-1641 to G in the nucleotide sequence depicted in SEQ ID NO:4.