An object of the present invention is to provide a method for determining whether a subject suffers from malignant lymphoma or leukemia and an agent for treating and/or preventing the disease. The present invention relates to a method for assisting in determining whether a subject suffers from, or is likely to suffer from malignant lymphoma or leukemia, comprising: a detection step of detecting at least one of a fusion mutation of a DUX4 gene, an overexpression of a DUX4 gene, and a fusion mutation of an MEF2D gene and a determination step of determining that the subject suffers from or is likely to suffer from the disease when at least one of the fusion mutations or the overexpression is detected. Moreover, the present invention relates to a pharmaceutical composition comprising a DUX4 inhibitor as an active ingredient, for treating and/or preventing malignant lymphoma or leukemia in a subject having a fusion mutation of a DUX4 gene and an IGH or IGL gene and/or overexpression of a DUX4 gene.
