IL1RN and bacterial arthritis: Although none of the mRNA transcripts corresponding to the previously reported differentially abundant proteins satisfied the rigorous criteria for the differentially expressed mRNA used in this study, mRNA for neutrophil collagenase (MMP1/8) and interleukin-1 receptor antagonist protein (IL1RN) stand out as being consistently upregulated in the synovial fluid from joints with septic arthritis.