In support of the hypothesis that hypermethylation of EGLN1 contributes to the excessive production of red blood cells in CMS, EGLN1 inactivation in mice results in an overproduction of erythropoietin and polycythemia, and familial polycythemia in humans has been linked to EGLN1 mutations [164,165,166]. This evidence concerns the gene EGLN1 and familial polycythemia.