Yes,When Receiver received out of order frame.
In selective repeat protocol, a frame can be received out of order and further those frames are sorted to maintain a proper sequence of the frames. While sorting, if a frame number is skipped, then receiver recognize that a frame is lost and it sends NAK for that frame to the sender.(for example if frame No.-2 lost then Receiver send NAK2 to Sender) After receiving NAK for the lost frame the sender search that frame in its window and retransmits that frame(Frame No.-2).