MX1 and avian influenza: Some of the most highly up-regulated genes seen during the duck host response to H5N1 include RSAD2 (viperin) which encodes an interferon-inducible antiviral protein involved in TLR7-dependent production of IFNB, MX1 which is known to have antiviral activity against avian influenza and DDX58 (RIG-I) which encodes a pattern recognition receptor for viruses.