There is amino acid variability between the erythropoietin produced by pig and NHP/human kidneys, which may contribute to the gradual development of normocytic anemia in NHPs that receive life-sustaining porcine allografts (56) as a molecular incompatibility between porcine erythropoietin and the NHP erythropoietin receptor exists. This evidence concerns the gene EPO and anemia (phenotype).