A deformable member can be used in a well tool for use in downhole oil/gas wells. In one embodiment, a deformable member is described which is deformable between undeformed and deformed positions, and comprises a generally hollow cylindrical body defining a wall. The wall includes three circumferential lines of weakness in the form of grooves, with two grooves provided in an outer surface of the member wall and the other groove provided in an inner surface. The member is deformed outwardly by folding about the lines of weakness and is used in particular to obtain sealing contact with a tube in which the member is located.