Flake drained tuna in small bowl.
Gently mix in green beans.
Combine blue-cheese dressing with sour cream.
Alternate two layers each of tuna and sour-cream mixtures in a 3-cup casserole.
Mix just enough to allow sour cream to seep to bottom of casserole.
Top with breadcrumbs mixed with butter.
Bake in a 350° F.oven for 15 to 20 minutes.