Smalberg et al. [19] recently demonstrated that the 46/1 haplotype is associated with the development of JAK2 V617F-positive splenic vein thrombosis (SVT), but the existence of JAK2 V617F-negative SVT patients also indicates an important role for the 46/1 haplotype in the etiology and diagnosis of SVT-related MPNs. The gene discussed is JAK2; the disease is Supraventricular tachycardia.