C17orf99 encodes interleukin-40 (IL-40), a novel 27-kDa B cell-associated cytokine involved in humoral immune regulation 1. IL-40 is expressed in bone marrow, fetal liver, and peripheral B cells upon activation, with expression further induced in the mammary gland during lactation 1. The cytokine plays a critical role in IgA production and mucosal immunity; C17orf99-deficient mice exhibit reduced serum and gut IgA levels, smaller Peyer's patches, and altered microbiome composition 1. Beyond immune homeostasis, IL-40 has emerged as a pathogenic mediator in inflammatory diseases. Serum IL-40 levels are significantly elevated in rheumatoid arthritis (RA) patients, correlating with disease activity, autoantibodies (rheumatoid factor and anti-CCP), and neutrophil extracellular trap markers 2. IL-40 decreases following B cell-depleting rituximab therapy but not TNF inhibition, suggesting B cell-dependent production 2. In osteoarthritis, IL-40 accumulates in synovial fluid and cartilage, promoting chondrocyte pro-inflammatory cytokine secretion and matrix metalloproteinase expression 3. Genetic studies identify the rs2004339 A/G intergenic variant as associated with increased RA risk, particularly in women 4. Additionally, C17orf99 mRNA is downregulated but hypermethylated in acute myeloid leukemia patients, with the rs2004339 AA genotype associated with increased AML susceptibility 5. These findings indicate IL-40 functions in both normal B cell immunity and pathogenic inflammatory responses across multiple diseases.