The existence of a protein product for ORF4 has not been confirmed. In the MNV virion structure, the capsid, like that of human noroviruses, consists of 90 dimers of VP1 [30] . There are differences between the MNV virion and previously reported VLP structures. The MNV protruding domain is lifted off the shell domain by approximately 16 Angstroms and rotated approximately 40 degrees in a clockwise fashion, forming interactions at the P1 base in an infectious virion that have not been observed previously. The existence of these novel aspects of the structure are consistent with the hypothesis that MNV may undergo a capsid maturation process [30] .