DPP4 and periodontitis: Other than ACE2, BMAL1, CD147, FURIN, and TMPRSS2, more genes, including CD26(DPP4), IFITM3, HLA, ABO, SLC6A20, GSTT1-M1, and DBP [18], have also been implicated in SARS-CoV-2 and periodontitis.