The purpose of this study is to find out if an investigational drug called setmelanotide (RM-493) can help control body weight in people with certain genetic modifications in their genes that play a key role in the regulation of body weight. Males and females, age 6 to 65, with obesity and certain genetic variants (MC4R pathway) may be eligible to participate. If you are confirmed eligible for the study, then participation will last up to 64 Weeks. Participants will complete Screening (Up to 8 Weeks), Double-Blind Treatment (Up to 52 Weeks), and Follow-Up Period (4 Weeks). Participants will complete research procedures including physical exams, blood draws, ECGs, and questionnaires. Participant will also need to self-inject the study drug once daily for the duration of their participation.