JAK2 and myeloproliferative neoplasm: This view is supported by the observation that the gain-of-function Tumerous-lethal (Tuml) allele of Drosophila JAK inhibits heterochromatin formation and reduces HP1:STAT association [5,53] – a characteristic also shared by the gain-of-function JAK2 V617F allele associated with the majority of human myeloproliferative neoplasms [54].