Northern blot analysis of human Jak2 expression. Commercial membranes containing 2 μg Poly A⁺ mRNA per lane of various human tissues (Clontech) were probed with a 400-bp cDNA corresponding to the 3′ end of human Jak2. Hybridization and washing of the membranes were performed according to the manufacturer's specifications (Clontech). β-Actin was used for comparison of RNA loading.