This qualitative case study aimed to investigate prospective middle school mathematics teachers' instructional responses based on the students' correct and incorrect functional thinking within the context of pattern generalization. The data were collected from thirty-two prospective teachers through a written task and semi-structured interviews and analyzed with open coding. The study's findings revealed that most prospective teachers could support the functional thinking of the student who had an incorrect solution. However, they could not extend the student's functional thinking of those who reached the correct solution. Instead, they asked the student to do another similar drill or provided a general response to the student with the correct solution without extending their functional thinking.