Objective The strategy to investigate an individual miRNA function in vivo is to delete or to overexpress it in animals.It is demonstrated that individual microRNAs can exert critical control over mammalian differentiation processes in vivo.The modified mouse strains with the ablation of certain individual miRNAs show that the specific mouse cells tend to malfunction.Meanwhile,the conditional miRNA knockout mice generated by Cre-lox technology are able to ensure that the miRNA function could be studied in a tissue-and temporal-specific manner.Noticeably,some miRNAs exist in a cluster manner in animal genomes,functioning similarly in the posttranscriptional regulatory network within animal body.The whole-cluster and respective knockout mice,having interestingly different phenotypes,are also produced to cast light on explaining their essential roles in the control of special gene expression and function.