We screen the genes that essential or related to zebrafish embryonic development by using ENU mutagen.By screening we got a recessive mutant, named as Tsu1363.This mutant shows normal development till 36hpf.The defects begin after 36hpf,including small heads and eyes, CNS necrosis, thin bending body, rounder yolk with thin extension, and underdeveloped gut.First, we identified the mutant gene is bysl by mapping technology.The bysl gene of Tsu1363 occurs a nonsense mutation (the Thymidine at 951 mutates to the Adenine), producing a premature stop codon.We can successfully rescued the mutant phenotype by injecting bysl mRNA at 1-cell stage.Moreover, two bysl morpholinos which inhibit bysl translation can recur the defects showed in Tsu1363 mutants.These experiments confirm the bysl gene is the mutant gene in Tsu1363.Then, we detected the bysl expression pattern by using in situ hybridization.Bysl mRNA is maternally expressed and after 36hpf, it mainly expressed at eyes and mid-hindbrain boundary.This indicates bysl may have different roles at different stages.It has been reported that enp1,the homologous gene of bysl in yeast, is required for pre-rRNA processing and 40S subunit synthesis.And bysl also plays a crucial role in mammalian preimplantation development.However, it has never been reported for its function in the development of eye and brain.As the mutant phenotype shows and the mRNA expression pattern, bysl may also play an important role in nervous system development.Further research need to be done.