A seal (10) is used for closing-off a proximal-side access port of an access instrument into a body. It has a cap (12) having a wall (14) covering the access port and a circumferential collar (16), which can be put over an edge of the access port. At least two approximately circular-segment-shaped openings are present in the wall (14), wherein a flexible dome sits on each opening and, on the proximal side, has, compared to the circular-segment-shaped openings, a smaller entry port (26, 28).