 As of 2014 , the <unk> web site , an archive of miRNA sequences and annotations , listed 28 @,@ 645 entries in 233 biologic species . Of these , 1 @,@ 881 miRNAs were in annotated human miRNA <unk> miRNAs were predicted to have an average of about four hundred target mRNAs ( affecting expression of several hundred genes ) . Friedman et al. estimate that > 45 @,@ 000 miRNA target sites within human mRNA <unk> are conserved above background levels , and > 60 % of human protein @-@ coding genes have been under selective pressure to maintain pairing to miRNAs . 
