Collectively, these data show that heterozygous endogenous JAK2V617F expression in hematopoietic cells leads to hyperplasia of mature and maturing erythroid, granulocytic, and megakaryocytic cells in blood and hematopoietic tissues, an MPN phenotype that may not be influenced by the P53 genetic inactivation. The gene discussed is TP53; the disease is myeloproliferative neoplasm.