As part of a post-approval commitment, the Korean health authority requests a study to characterize safety and effectiveness in patients treated with Koselugo (Selumetinib), an oral selective inhibitor of MAPK kinase (MEK) 1 and 2, by physicians in routine clinical practice settings. This study is designed to assess the known safety profile or identify previously unsuspected adverse reactions and evaluate the effectiveness of Koselugo under conditions of routine daily medical practice in Korea. This study will provide information on the Korean patient population that is treated with the study drug.