The function of insulators is directional, a discovery made through fruit fly experiments. Inserting the transposable element gypsy into the yellow gene locus on Drosophila melanogaster can deactivate the y gene in some tissues, while in others, the y gene remains active. This is due to an insulator sequence at one end of the gypsy transposable element. Different effects on gene activity are observed when gypsy is inserted at various positions in the y gene locus. This is because the activity of the y gene is regulated by four enhancers. When an insulator is inserted upstream of the promoter, it blocks the activation of the gene in wing blade and body cuticle tissues (from upstream enhancers), but does not block the expression of the y gene in bristle and farsal claw tissues (from downstream enhancers).