Of interest in this regard, Bettegowda et al. [31] demonstrated the ability to detect any ctDNA (albeit without sequencing the genes in the ctDNA) in over 75% of 640 patients with various cancer types, but in less than 50% of primary brain, renal, prostate, or thyroid cancers, suggesting that physical obstacles such as the blood-brain barrier and mucin could prevent ctDNA from entering the circulation [32]. Here, MUC5AC is linked to cancer.