MMP13 and chronic periodontitis: AP-1 regulates MMPs,31 with c-FOS and c-JUN heterodimers specifically regulating MMP13,32 possibly through the transcription factor ATF3 activation.33 Accordingly, downregulation of JUN and FOS family members coincided with decreased MMP13 and ATF3 expression (Supplementary Table 1), suggesting reduced AP-1-mediated process during chronic periodontitis.