The fruitless gene (fru) is a Drosophila melanogaster gene that encoes several variants of a transcription factor protein. Normal fruitless function is required for proper development of several anatomical structures necessary for courtship, including motor neurons which innervate muscles needed for fly sexual behaviors. The gene is not present in mammals. Nor do many other Drosophilid flies contain clear homologs, suggesting that the specifics of fruitless function are unlikely to carry over to other animals. Despite the narrow applicability of research on the fruitless gene, it has historically served as a popular example of how changes in gene expression can lead to changes in behavior, and increasingly as case study for genetic control of nervous system development. Research on fruitless has received attention in the popular press, since it is seen to be connected to the issue of the genetics of human sexual orientation, and behaviors such as gender-specific aggression.