A gas turbine engine (20; 201) comprising: a first case structure (220; 320; 420; 520); a second case structure (222; 322; 422; 522); and an isolation seal (228; 330; 430; 530) arranged between the first case structure (220; 320; 420; 520) and the second case structure (222; 322; 422; 522) and configured to provide sealing engagement therebetween, the isolation seal comprising: a first interface member (334; 434; 534) configured to be fixedly attached to the first case structure (220; 320; 420; 520), the first interface member being a full-hoop structure; a housing (340; 440) configured to be mounted to the second case structure (222; 322; 422; 522); a connector pin (338; 438; 538) arranged within the housing (340; 440) and moveable relative to the housing; a second interface member (336; 436; 536) located on an end of the connector pin (338; 438; 538), the second interface member being a full-hoop structure; and a biasing member (342; 442) arranged within the housing (340; 440), the biasing member (342, 442) configured to urge the second interface member (336; 436; 536) away from the housing (340; 440); wherein the first interface member (334; 434; 534) and the second interface member (336; 436; 536) are engageable to form a seal therebetween; and wherein the isolation seal provides sealing engagement between the first interface member (334; 434; 534) and the second interface member (336; 436; 536) when there is relative movement between the first case structure (220; 320; 420; 520) and the second case structure (222; 322; 422; 522), the relative movement comprising each of axial relative movement, radial relative movement, and angular relative movement. a first case structure (220; 320; 420; 520); a second case structure (222; 322; 422; 522); and an isolation seal (228; 330; 430; 530) arranged between the first case structure (220; 320; 420; 520) and the second case structure (222; 322; 422; 522) and configured to provide sealing engagement therebetween, the isolation seal comprising: a first interface member (334; 434; 534) configured to be fixedly attached to the first case structure (220; 320; 420; 520), the first interface member being a full-hoop structure; a housing (340; 440) configured to be mounted to the second case structure (222; 322; 422; 522); a connector pin (338; 438; 538) arranged within the housing (340; 440) and moveable relative to the housing; a second interface member (336; 436; 536) located on an end of the connector pin (338; 438; 538), the second interface member being a full-hoop structure; and a biasing member (342; 442) arranged within the housing (340; 440), the biasing member (342, 442) configured to urge the second interface member (336; 436; 536) away from the housing (340; 440); wherein the first interface member (334; 434; 534) and the second interface member (336; 436; 536) are engageable to form a seal therebetween; and wherein the isolation seal provides sealing engagement between the first interface member (334; 434; 534) and the second interface member (336; 436; 536) when there is relative movement between the first case structure (220; 320; 420; 520) and the second case structure (222; 322; 422; 522), the relative movement comprising each of axial relative movement, radial relative movement, and angular relative movement. a first interface member (334; 434; 534) configured to be fixedly attached to the first case structure (220; 320; 420; 520), the first interface member being a full-hoop structure; a housing (340; 440) configured to be mounted to the second case structure (222; 322; 422; 522); a connector pin (338; 438; 538) arranged within the housing (340; 440) and moveable relative to the housing; a second interface member (336; 436; 536) located on an end of the connector pin (338; 438; 538), the second interface member being a full-hoop structure; and a biasing member (342; 442) arranged within the housing (340; 440), the biasing member (342, 442) configured to urge the second interface member (336; 436; 536) away from the housing (340; 440); wherein the first interface member (334; 434; 534) and the second interface member (336; 436; 536) are engageable to form a seal therebetween; and wherein the isolation seal provides sealing engagement between the first interface member (334; 434; 534) and the second interface member (336; 436; 536) when there is relative movement between the first case structure (220; 320; 420; 520) and the second case structure (222; 322; 422; 522), the relative movement comprising each of axial relative movement, radial relative movement, and angular relative movement.