AbstractInterleukin 7 receptor (il7r), a transmembrane receptor, belongs to the type I cytokine receptor family. Il7r is involved in the pathogenesis of neurodegenerative disorders, such as multiple sclerosis. Targeted knockdown of il7r leads to delayed myelination, highlighting the potential role of il7r in the development of the nervous system. Zebrafish is an ideal model for the study of neurogenesis; moreover, the il7r gene is highly conserved between zebrafish and human. The aim of the present study was to investigate the novel function of il7r in neurogenesis. First, an il7r−/− homozygous mutant line was generated by clustered regularly interspaced short palindromic repeats (CRISPR)-associated 9 (CRISPR/Cas9) technology. Second, the gross development of il7r−/− mutants revealed remarkably smaller eyes and delayed retinal neurodifferentiation. Third, microarray analysis revealed that genes associated with the phototransduction signalling pathway were strongly down-regulated in il7r−/− mutants. Finally, the results from behavioural tests indicated that visual function was impaired in il7r−/− mutant larvae. Overall, our data demonstrate that a lack of il7r retards the development of the retina. Thus, il7r is an essential molecule for maintaining normal retinal development in zebrafish.