JAK2 and myeloproliferative disorder: The haplotype 46/1 (“GGCC”), a specific constitutional JAK2 gene haplotype that increases the risk of MPN (including both increased susceptibility to develop JAK2 mutated MPNs and less often non-JAK2 mutated MPNs) [26], is also found to be a predisposition factor for IBD [28,30].