That information is relatively well known to those familiar with the field of clonal hematopoiesis, and was likely omitted for brevity. It is not reasonable to expect lay public level of background information to be included in every discipline's scientific publications.

I'll point you to the two seminal papers in the field of CH though if you are interested. These were published back-to-back in the same issue of NEJM in 2014 (interestingly from separate competing groups at Harvard).

https://www.nejm.org/doi/full/10.1056/nejmoa1408617 https://www.nejm.org/doi/full/10.1056/nejmoa1409405