With regard to the Th1 subset, leukocytes or CD4-1+ lymphocytes from many bony fish species such as zebrafish, rainbow trout, grass carp, tilapia, and mandarin fish have been reported to express IFN-γ at the mRNA level, and its transcription is markedly induced by pathogenic infection or PAMP stimulation [27,32,37,50,59]. The gene discussed is IFNG; the disease is infection.