Cytokine mRNA expression in zymosan A injected knee joints of ob/ob and +/? mice. Cytokine mRNA expression during acute and chronic phases of zymosan-induced arthritis was investigated on total RNA isolated from the knee joints of five ob/ob (black columns) and five +/? (white columns) mice on (a) day 1 and (b) day 14 after injection of zymosan A. Cytokine mRNA expression levels were assessed by RNase protection assay (see Materials and methods section), quantified by phosphor-imaging and normalized for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) expression. Data are expressed as a ratio of cytokine to GAPDH mRNA expression and represent means ± SEM (n = 5) of values obtained for detectable cytokines in zymosan A injected knees. Cytokine expression in saline injected control knees was low or undetectable in all animals.