To analyze the allelic distribution of the identified mutations with potential functional effects on AP-1-dependent gene expression in RA (i.e., mutations detected in RA patients causing an aa exchange: fos73, fos125, jun202, and jun236), genotyping was performed in RA (n = 298), OA (n = 288), and NC (n = 484) whole blood DNA samples by single base extension and matrix assisted laser desorption ionization–time of flight (MALDI-TOF) multiplex analysis. This evidence concerns the gene FOS and rheumatoid arthritis.