MicroRNAs (miRNAs) represent a class of non-coding RNA molecules playing pivotal 
roles in cellular and developmental processes. miRNAs modulate the expression of 
multiple target genes at the post-transcriptional level and are predicted to 
affect up to one-third of all human protein-encoding genes. Recently, miRNA 
involvement in the adaptive and innate immune systems has been recognized. 
Rheumatoid arthritis serves an example of a chronic inflammatory disorder in 
which miRNAs modulate the inflammatory process in the joints, with the potential 
to serve as biomarkers for both the inflammatory process and the potential for 
therapeutic response. This review discusses the investigations that led to miRNA 
discovery, miRNA biogenesis and mode of action, and the diverse roles of miRNAs 
in modulating the immune and inflammatory responses. We conclude with a 
discussion of the implications of miRNA biology in rheumatoid arthritis and 
other autoimmune disorders.