C3, C4 and C5 knockout mice together with Crry, DAF, C3aR and/or C5aR1 and C5aR2 knockout can be used for defining the role of complement in general and C3a-C3aR and C5a-C5aR signaling in the pathogenesis of severe COVID-19 and long COVID-19 [136, 146–151]. The gene discussed is C5AR2; the disease is COVID-19.