This study is checking if a new medicine called cemsidomide is safe and how well it works for people with certain types of blood cancer, like Non-Hodgkin's Lymphoma (NHL) and Multiple Myeloma (MM). Some people with MM will also take another medicine called dexamethasone. The study will test different doses to see which is best.

• The study is for adults 18 and older with certain types of NHL or MM that haven't responded to past treatments.
• Participants will need to have specific medical tests and meet health requirements to join.
• People who join will need to follow certain rules like not donating blood or sperm during and a month after the study.

To join, you need to be healthy enough, not have certain other illnesses, and be willing to follow all the study's rules. The study will provide detailed instructions, and there might be some risks, so it's important to talk to your doctor about whether this study is right for you.