I have an SDHB gene mutation: it was discovered after two younger brothers developed tumours in childhood (pheochromocytoma/paraganglioma). Further screening revealed a silent tumour in my Aunt, a tumour in another sibling, and a brother has, sadly, recently had a recurrence (that's the criteria for it being malignant). My maternal grandmother died of Gliobastoma Multiforme back in 1985, so that is highly indicative of where it came from.
There are several hundred of us in the world with this mutation, and we are apparently one of 2-3 families in the U.K. However, it's very likely that there are people living with silent mutations that will not, or have not yet, manifested themselves.
My lifetime risk of cancer is about 95%, and I am screened annually (MRI, bloodwork, pee in a big bottle for 48 hrs ... urgh). I am 34, and presently symptom free. It may stay that way.
It is not like the BRAC gene in that I can have an operation (the cells from which tumours spring are all over the nervous system, so would be filleted in manner of fish!).
On a daily basis, I don't give it much thought because, well, what can you do? Nothing: I trust that screening will detect anything before it becomes a massive problem, and there is no reason to suspect it will be diagnosed at a very advanced stage.