Additional Study Information: You are being asked to participate in this study because you have indolent SM (ISM) or because you have Monoclonal mast cell activation syndrome (mMCAS). Currently, there are no available targeted therapies for patients with ISM or mMCAS. The study will test the drug BLU-263. BLU-263 is a new study drug that is being developed for treating people like you, with ISM and/or mMCAS. BLU-263 is not yet approved for treating ISM and/or mMCAS by the Food and Drug Administration (FDA). BLU-263 is the first study drug in a group of drugs that prevents a specific DNA change called KIT D816V. The goal is to find a safe and acceptable dose in people with ISM and/or mMCAS and to find how the body processes the study drug at different dose levels. The drug company also wants to find out what effects, good and bad, BLU-263 has on participants in this study.