Essentially, these animals exhibit features of Hif2α overexpression, which are also manifested in patients with Chuvash polycythemia (Ang et al., 2002), and other forms of familial polycythemia caused by “gain of function” HIF2α mutations (Percy et al., 2008), or inactivating mutations in pVHL, a negative regulator of HIF2α stability (Percy et al., 2006). This evidence concerns the gene EPAS1 and Chuvash erythrocytosis.