[Problem] To describe a novel gene mutation that is a cause of breast cancer, and to thereby provide: a method of detecting breast cancer in test subjects by detecting the genetic mutation or the protein associated with the genetic mutation; a method of diagnosing breast cancer in test subjects; and a primer set, probe and detection kit therefor. [Solution] A method for detecting the presence of breast cancer in test subjects which includes a step for detecting the mutation of the 82nd asparagine in the tyrosine kinase domain of fibroblast growth factor receptor 4 (FGFR4) to asparginic acid or lysine in a sample obtained from a test subject.
