A Promethease user who has this rs12340895(G;G) genotype has reported that they have mastocytosis, a myeloproliferative neoplasm (MPN). But don't panic! Mastocytosis is listed as a rare disease by the Office of Rare Diseases (ORD) of the National Institutes of Health, which means mastocytosis affects less than 200,000 people in the entire US. To put it another way, 5 to 10 new cases are reported annually per million people. Since this rs12340895(G;G) genotype is carried by many people (e.g. ~10% of all Caucasians, or, over 30 million Americans), it's clear the overwhelming majority do not develop mastocytosis.