Poliomyelitis is a disease of the central nervous system. However, CD155 is believed to be present on the surface of most or all human cells. Therefore, receptor expression does not explain why poliovirus preferentially infects certain tissues. This suggests that tissue tropism is determined after cellular infection. Recent work has suggested that the type I interferon response (specifically that of interferon alpha and beta) is an important factor that defines which types of cells support poliovirus replication. In mice expressing CD155 (through genetic engineering) but lacking the type I interferon receptor, poliovirus not only replicates in an expanded repertoire of tissue types, but these mice are also able to be infected orally with the virus.