The vast majority of commercially available tissue valves are xenograft valves derived from animals. Usually, the valves are porcine, meaning from a pig, or bovine, meaning from a cow. The porcine heart is most similar to the human heart and therefore represents the best anatomical fit for replacement. The tissue of the pericardial sac is particularly well suited for a valve leaflet due to its extremely durable physical properties. Before implantation, the tissue undergoes multiple chemical treatments to make it suitable for replacement in the human heart. The tissue is sterilized so that the biological markers are removed, thereby eliminating a response from the host's immune system.