MicroRNAs （miRNAs） are endogenous -22 nucleotide noncoding RNAs that regulate the expression of complementary messenger RNAs （mRNAs）. Thousands of miRNA genes have been found in diverse species, and many of them are highly conserved. With the miRNA roles identified in nearly all aspects of biological processes, evidence is mounting that miRNAs could represent a new layer of regulatory network, and their regulatory effect might be much more pervasive than previously suspected. Here we focus on the posttranscriptional level gene regulation of miRNAs in animals and review how the miRNAs act to sustain and shape up the expression profiles of specific cell types; how the miRNAs integrate into the existing gene regulatory networks; and how the miRNAs influence the evolution of 3＇UTR of mammalian mRNAs.
