There is a compelling need to strengthen professional learning (PL) regarding systems thinking, since it crosses all disciplinary boundaries and is a crosscutting concept in the Next Generation Science Standards. Yet research has shown that effectively deploying systems thinking in the classroom is challenging both for students and for teachers. The Exploring the Integration of Systems Thinking into Biology (ExIST) project engages teachers in PL that seeks to enhance teachers’ perception of the important role of systems in biology and understand how game design can support students’ systems thinking. Building on prior research, the PL is informed by the synergy between systems thinking in biology and systems thinking as inherent in game design. This paper presents a case study of how two middle school science teachers, engaged in systems thinking and a game design task, worked together to modify a Scratch game during a PL workshop. Their goal was to use the modified game during classroom enactment of a systems and game design unit. Our study focuses on what the teachers learned while they were remixing the game. Findings suggest that remixing the Scratch game helped the teachers clarify their instructional goals for their students. But it also helped the teachers themselves learn about systems dynamics, how Scratch works, and the utility of game design as a way to model real-world phenomena. We conclude with lessons learned about the design of PL based on game design.