This gene locus lies within 60 kb of the highly homologous gene locus Il17A. In order to generate a double mutant, ES cells heterozygote for the Il17atm1Yiw null allele were retargeted with a second vector to the Il17f locus. A hygromycin resistance gene replaced exons 2 and 3 of the allele that was in trans with the Il17atm1Yiw allele. The two mutant loci were bred apart and gene inactivation of just the Il17f allele was confirmed by RT-PCR of activated splenocytes collected from homozygote mice.