MX1 and infection: Interestingly, in the gilthead seabream brain and head-kidney upon in vivo infection with NNV, the phagocytosis, the cell-mediated cytotoxic activity and the mx transcription started very early upon the infection and stayed at high up to 15 days, while in the European sea bass brain these activities and the mx transcription was up-regulated at day 1 upon infection and quickly decreased to control levels [49].