Indeed, changes in the concentrations of proenkephalin and substance P have been reported in COVID-19 patients: the plasma concentration of proenkephalin is increased in COVID-19 patients [54], and the levels of substance P and its neurokinin receptors have been found to be increased in olfactory neurons, with the latter being proportional to the residual olfaction [76]. The gene discussed is TAC1; the disease is COVID-19.