This study is testing a medicine called elenestinib (BLU-263) to see if it helps people with a condition called indolent systemic mastocytosis (ISM). ISM is a condition where certain cells in the body, called mast cells, grow too much and cause symptoms like redness or itching. Elenestinib is being compared to a placebo (a pill with no medicine) to see if it works better. The study is in two main parts, and people in the study will get either elenestinib or a placebo, along with their regular treatment. In the last part, everyone gets elenestinib to see how well it works. People who already tried a similar medicine can join a special part of the study.

• The study will have three parts, and you may receive treatment for a long time.
• Requirements include stable symptom management before joining.
• Some people may not be able to join if they have other health issues.