The function of insulators is directional, a finding discovered in fruit fly experiments. Inserting the transposon gypsy into the yellow gene locus y of fruit flies (D. melanogaster) causes the y gene to become inactive in some tissues, while it remains active in others. This is due to an insulator sequence at one end of the gypsy transposon. When gypsy is inserted at different positions in the y gene locus, it has varying effects on gene activity. 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 gene activation in wing blade and body cuticle tissues (from enhancers upstream), but does not block the expression of the y gene in bristle and farsal claw tissues (from enhancers downstream).