This study is for adults with **relapsed or refractory multiple myeloma (RRMM)**, a type of blood cancer that has returned or doesn't respond to treatment. The focus is on a new medicine called KTX-1001, which is a **small molecule inhibitor** targeting MMSET, a protein linked to cancer cell growth. The study has two parts: Part A tests different doses for safety, lasting 28 days, while Part B sees how well the chosen dose works in specific patients. Participants must be 18 or older and have tried at least three previous treatments. They should also have specific genetic changes in their cancer. Some past treatments or health conditions may prevent joining the study. **Key Points:** - **Study Duration:** The first phase is 28 days, with further treatment based on the initial results. - **Eligibility:** Must have specific genetic markers and have tried at least three prior treatments. - **Exclusions:** Recent major surgery, specific health conditions, and certain past treatments may disqualify participation. This study helps understand the safety and effectiveness of KTX-1001 in treating RRMM.