MMP-13 expression in joints during arthritis development. (A) Semi-quantitative polymerase chain reaction analysis of RNA derived from hind paws of mice at days 0, 3, and 8 and 14 showing matrix metalloproteinase (MMP)-13 expression (320 base pairs) induced by injection of arthritogenic serum in control C57BL/6 mice. C, cDNA from RNA isolated from mouse bone serving as positive control; m36b4, ribosomal protein PO transcript levels served as control for quality and quantity of tested RNA. (B) Quantification of polymerase chain reaction amplified MMP-13 mRNA. **P <0.01.