Background: Mutants of the serine protease, mannose-binding lectin associated-protease-3 (MASP-3), are associated with Carnevale, Mingarelli, Malpuech and Michels (3MC) syndrome. Results: The lack of activity and the structure of the G666E mutant of MASP-3 reveals a molecular basis for 3MC syndrome. Conclusion: Mutants of MASP-3 associated with 3MC syndrome are inactive. Significance: The catalytic activity of MASP-3 is required for normal developmental processes.