KLLN and breast cancer: A well‐known gene mutation‐related disease is Cowden syndrome, which involves splice variants of the tumor suppressor gene PTEN and mutations of CHEK2.10, 11 Patients with Cowden syndrome have an increased risk of both BC and TC, and the prevalence of Cowden syndrome is approximately 1 in 200 000.12 Mutations in the succinate dehydrogenase gene (SDHx) and KLLN were recently reported to cause Cowden syndrome and Cowden‐like syndrome13, 14 and to thus contribute to the co‐occurrence of BC and TC.15