There is a gene called NRAS and we know now that that is the most common cause of CMN of any size and any number. Overall, in one big cohort it causes 67% of cases. In medicine, numbers are always a little bit inaccurate could be a bit less than that could a bit more than that.
A gene called BRAF has also been found as being a cause for 7% CMN, much less common than the NRAS mutation but it is definitely a cause.
In other cases, 25% of cases researchers have described single cases who have had different cause. In each of those cases so far only one person had that cause, so we don’t know whether there’s going to be many other possible causes.