MicroRNAs(miRNAs),a class of non-coding RNAs,approximately 22–24 nucleotides long.Through interacting with 3’ untranslated region(UTR)of target genes to regulate the stability of their mRNAs or inhibit their translation,miRNAs modulate numerous biological processes including circadian rhythms,biological rhythms with a period of approximately 24 hours.To date,investigation of the circadian clock primarily focused on transcriptional level and post-translational modification,little is known about post-transcriptional regulation.In mice,knockdown of miR-219 lengthened the circadian period.Here,using quantitative RT-PCR,we observed that zebrafish miRNA-219 is rhythmically expressed,with peak expression at ZT12(9 o’clock in the evening).We also generated transgenic fish lines containing luciferase report gene driven by the miR-219 promoter and showed miR-219 oscillates in vivo.Chromatin immunoprecipitation(ChIP)assays showed the Bmal1b protein binds to the E-boxes in the miR-219 promoter region.In Vitro cell transfection and over-expression experiments showed that miR-219 down-regulates bmal1b,rather than bmal1a or baml2.We also generated miR-219 mutants by CRISPR/Cas9,and are in process of characterizing their phenotypes.