CA9 and respiratory failure: For example, a clinical trial using carbonic anhydrase IV (CAIX) CAR-T cells for metastatic renal carcinoma had to be halted due to liver toxicity that was related to cross-reactivity to CAIX-positive bile duct epithelium.10 In another trial, a patient with metastatic colon carcinoma treated with a third-generation CAR targeting HER2 succumbed to respiratory failure caused by CAR-T cells recognising low levels of HER2 expressed on the lung epithelium.11