We identified fibroblast activation protein (FAP) as one such marker and provide proof-of-concept in cancer patients that molecular imaging of FAP with the 68Ga-DOTA-FAPI-46 PET tracer [16] is a promising strategy to quantify ‘CMS4 tumour load’ in CRC patients, and—possibly—to evaluate the response of these lesions to systemic therapy. This evidence concerns the gene FAP and cancer.