A bone anchor comprisinga shank (2) for anchoring in a bone or a vertebra anda receiving part (4) for receiving a rod (7) and for coupling the rod to the shank,the receiving part (4) comprising a recess (61) by means of which a base (62) and two free legs (63) extending from the base (62) are formed,the legs (63) each having a first portion (63a) extending from the base (62) of the recess (61) and a second portion (63b) extending from an end of the first portion (63a), wherein an internal thread (65) is formed in at least a section of each of the first and the second portion and wherein between the first portion (63a) and the second portion (63b) a breakaway portion (66) is provided that permits to break away the second portion (63b) from the first portion (63 a), andwherein elongate extension members (67, 67, 67") are provided on the legs, respectively, that extend from an end of the second portion (63b)and wherein a stiffness of the elongate extension members (67, 67, 67") is less than the stiffness of the first portion (63a) and/or the second portion (63b) of the legs (63).
