The intron between exon 1 and 2 of the zfh1 gene has been deleted. This selectively inhibits the expression of the 'circZfh1' circular RNA that is normally made by back-splicing of the second and third exons of zfh1 (and which encodes the CRAV, 'circular RNA-encoded antiviral protein' open reading frame) without significantly altering the expression level of the parental zfh1 mRNA.