New and Updated Topics and Pathways. In V81, Topics with new or revised pathways include Development ( Germ layer formation at gastrulation ), Immune system ( TAK1-dependent IKK and NF-kB activation ), and Signal Transduction ( Transcriptional activity of SMAD2/SMAD3:SMAD4 heterotrimer ). The SARS-CoV-2 life cycle has been updated to reflect our growing understanding of the life cycle.