As expected, Jak2V617F Flex/WT VE-cadherin-Cre mice, herein referred to as Jak2V617F HC-EC mice, in which VE-cadherin was expressed during early embryonic life in a precursor of both endothelial and hematopoietic cells (13), developed MPN, as attested by higher spleen weight (2.3%–5.7% of body weight vs. 0.3%–0.6% for littermate controls; P < 0.0001) and higher hemoglobin levels and platelet and WBC counts than in littermate controls (Figure 1, A–D). The gene discussed is CDH5; the disease is myeloproliferative neoplasm.