IL9R is an X-linked cytokine receptor located in the pseudoautosomal region of the sex chrX|Y that mediates immune responses by binding interleukin-9 and activating JAK-STAT signaling. Upon IL-9 engagement, IL9R triggers phosphorylation of JAK1, JAK3, and STAT proteins (including STAT1, STAT3, STAT4, and STAT5), leading to altered T cell differentiation, proliferation, and cytokine secretion. In acute myeloid leukemia, leukemia stem cells produce IL-9, which signals through IL9R on CD4+ T cells to activate a Th1-skewed phenotype that paradoxically supports tumor expansion through IFN-γ and TNF-α production 1. In rheumatoid arthritis, IL9R signaling sustains a PU.1-IL-9 positive feedback loop in Th9 cells, driving joint inflammation 2. Dysregulation of IL9R appears in atopic dermatitis, where IL-9 signaling potentiates IL-18-driven Th2 responses 3, and in preeclampsia, where reduced IL9/IL9R signaling impairs trophoblast proliferation and angiogenesis 4. Engineered T cell therapies exploit IL9R signaling to enhance CAR T cell persistence and anti-tumor activity in solid tumors 5, with IL-9R demonstrating superior JAK/STAT activation compared to orthogonal receptor variants 6.