M5A also has been used for PET imaging of many CEA-positive cancers, including colorectal cancer (n = 11), medullary thyroid cancer (n = 5), esophagogastric cancer (n = 2), and pancreatic cancer (n = 2).68 In a phase 1 trial of M5A labelled with Yttrium-90, it was shown that the humanized version of M5A led to decreased risk of immunogenicity (development of human antibodies against a drug or other molecule) compared with previous work with chimeric versions of M5A.69 Here, CEACAM5 is linked to medullary thyroid gland carcinoma.