In the numerous clinical trials, FKN is used as a potential biomarker in different neoplastic (NCT02774395, endometrial cancer; NCT04576429, melanoma; and NCT04253145, small-cell lung cancer), autoimmune (NCT05387473, osteoarthritis and NCT04995588, systemic sclerosis), neurodegenerative, and cognitive disorders (NCT04506073, Parkinson’s disease; NCT06337539), as well as infectious diseases (NCT06444893, COVID19 and NCT04870138, gonococcal infection). The gene discussed is CX3CL1; the disease is COVID-19.