Sem is a partner at Backstage Legal, a legal collective based in Amsterdam. His expertise particularly extends to the areas of employment law, intellectual property law and privacy law. Sem also has a fascination with complex social (legal) issues, including the climate crisis and AI. In 2023, Sem spoke many times in public on these topics, especially AI. Before becoming a lawyer, Sem worked as a legal teacher and pop musician. With his band Caesar, he played four times at ESNS, three times at Lowlands and twice at Pinkpop. Caesar released four albums and won a silver harp in 1998. With his project Six Def, Sem has regularly scored syncs, such as in US series Suits and Californication. Sem also holds many other positions, including board member of the Pop Coalition and supervisory board member of Le Guess Who?