CARD9 (Caspase Recruitment Domain Family Member 9) is a critical adapter protein that mediates innate immune responses, particularly against fungal pathogens. The protein functions downstream of C-type lectin receptors such as Dectin-1, forming signaling complexes that activate NF-κB and MAPK pathways to promote inflammatory cytokine production 1. CARD9 operates through the CBM (CARD9-BCL10-MALT1) complex, which can be regulated by deubiquitination via OTUD1 2. Beyond antifungal immunity, CARD9 plays essential roles in gut microbiota regulation by influencing tryptophan metabolism into aryl hydrocarbon receptor ligands, with Card9-deficient mice showing altered microbiota composition and increased colitis susceptibility 3. The protein also functions in neutrophil-mediated immune responses, where it promotes Th17 cell expansion and contributes to inflammatory diseases like ankylosing spondylitis 4. CARD9 polymorphisms are associated with increased susceptibility to fungal infections and inflammatory bowel disease, mechanistically involving impaired cytokine production and defective neutrophil function 5. The protein's diverse functions extend to regulating intestinal inflammation, with Card9-knockout mice showing increased susceptibility to colitis due to disrupted IL-22 production and altered gut microbiome metabolism 3.