RIGI and viral infectious disease: In addition, Calu-3 cells expressed many of the proteins required for the major signalling pathways activated during viral infection: cGAS and STING (DNA sensing); RIG-I, MDA5 and MAVS (RNA sensing); the Toll-like receptor (TLR) adaptor MyD88; the kinase TBK1; and the transcription factors IRF3 and STAT1/2 (Fig. 1B).