Progression of K/BxN serum transfer arthritis in peptidyl arginine deiminase 4 ( PAD4) wild type (WT) ( n = 29) and knockout (KO) ( n = 27) mice. WT and PAD4 KO mice were immunized with pooled K/BxN serum as described and monitored for signs of disease severity for 10 days. Paw swelling (A) is expressed as the average percent size increase of four paws and two ankles compared to day 0 measured by caliper. Clinical indexes (B) of 0 to 3 were assigned for each paw as previously described . Data are combined from three independent experiments and represented as the average ± standard error of the mean (SEM). No statistically significant difference in the progression of K/BxN disease was apparent between WT and PAD4 KO mice.