This transmembrane region contains several amino acid differences between the two viruses, including methionine at position 390 and two extra leucine residues in the envelope protein of SFFV-P [32], both of which are required for the development of polycythemia in mice and to render BaF3-EpoR cells Epo-independent [33,34]. This evidence concerns the gene ERVW-1 and polycythemia.