Detecting tumor DNA in blood is more difficult than detecting fetal DNA due to tumor DNA's self-origin and mutations.

“the problem you described earlier has two things going for it which is one you've got much more of it there let's just assume the number is percent but the other thing is half of that DNA is foreign that's right 50% of that DNA's from the father 50% from the mother so you have half foreign greater quantity if you have colon cancer and we are lucky enough to get the RNA or DNA of that colon cancer it might have 20 mutated genes it's effectively self DNA well it is self exactly it's tumor DNA a tumor yeah exactly so I mean that's a double whammy it's not just a needle in a haystack it's like a needle of hay in a haystack”