Familial ET. In this family, JAK2 (V617F) was a somatically acquired mutation found in circulating granulocytes (with variable values for mutant allele burden in the different patients) but not in circulating T lymphocytes. Familial ET must be distinguished from hereditary thrombocytosis, a Mendelian genetic disease attributable to germ line mutations of JAK2, MPL, or THPO.