The sequences of twitchin and titin identify a superfamily of muscle proteins whose functions are not completely understood. In spite of their shared structural features, twitchin and titin appear to differ in function. Genetic and molecular evidence suggests that twitchin has a regulatory role in muscle contraction, whereas it has been proposed that titin has a structural function. We report here that Drosophila has a single-copy gene containing the two-motif amino acid sequence pattern that characterizes twitchin and titin. This gene appears to encode projectin, a muscle protein that is thought to play a structural role in asynchronous flight muscle but may have a role like that of twitchin in synchronous muscle. Thus Drosophila appears to be a case where the apparently diverged functions of twitchin and titin are encoded by a single gene.