This study is for participants with symptoms of mast cell activation (SMAC). The primary purpose of this study is to learn about clonal mast cell diseases. Clonal mast cell diseases are hard to diagnose because symptoms are not specific, and they can overlap with other diseases. The tools currently used by doctors to look for clonal mast cell diseases in the blood may not identify all patients. This study is being done to develop an investigational blood test that looks for a change in a gene called KIT. "Investigational" means it has not been approved by the United States Food and Drug Administration (FDA). Two types of blood tests will be compared against each other. Participants in this study can expect to be in this study for about 6 months.