Food web and gene regulatory networks (GRNs) are large biological networks,both of which can be analyzed using the May-Wigner theory.According to the theory,networks as large as mammalian GRNs would require dedicated gene products for stabilization.We propose that microRNAs (miRNAs) are those products.More than 30％ of genes are repressed by miRNAs,but most repressions are too weak to have a phenotypic consequence.The theory shows that (i) weak repressions cumulatively enhance the stability of GRNs,and (ii) broad and weak repressions confer greater stability than a few strong ones.Hence,the diffuse actions ofmiRNAs in mammalian cells appear to function mainly in stabilizing GRNs.The postulated link between mRNA repression and GRN stability can be seen in a different light in yeast,which do not have miRNAs.Yeast cells rely on non-specific RNA nucleases to strongly degrade mRNAs for GRN stability.The strategy is suited to GRNs of small and rapidly dividing yeast cells,but not the larger mammalian cells.In conclusion,the May-Wigner theory,supplanting the analysis of small motifs,provides a mathematical solution to GRN stability,thus linking miRNAs explicitly to ‘developmental canalization’.