BACKGROUND: The fibronectin type 3 and ankyrin repeat domains 1 gene, Fank1, is 
an ancient, evolutionarily conserved gene present in vertebrates. Short-hairpin 
RNA (shRNA)-based knockdown transgenic mice have oligospermia caused by an 
increase in apoptotic germ cells. In this study, we investigated the in vivo 
function of Fank1.
METHODS: In this study, we generated Fank1-knockout mice using the CRISPR/Cas9 
system. We then investigated the phenotype and in vivo function of Fank1. Testes 
and epididymis tissues were analyzed by histological and immunofluorescence 
staining. Apoptotic cells were analyzed in terminal deoxynucleotidyl transferase 
dUTP nick end-labeling assays. Fertility and sperm counts were also evaluated. 
The GTEx database were used to assess gene expression quantitative trait loci 
and mRNA expression of candidate genes and genes neighboring single nucleotide 
polymorphisms was analyzed by quantitative RT-PCR.
RESULTS: In contrast to the Fank1-knockdown model, no significant changes in 
epididymal sperm content and the number of apoptotic cells were observed in 
Fank1-/- homozygotes. In addition, a different pattern of Dusp1, Klk1b21 and 
Klk1b27 mRNA expression was detected in Fank1-knockout testis. These results 
reveal differences in the molecular changes between Fank1-knockdown mice and 
Fank1-knockout mice and provide a basic resource for population genetics 
studies.