FLT3LG (fms-related receptor tyrosine kinase 3 ligand) is a hematopoietic cytokine that stimulates proliferation of early hematopoietic cells by activating FLT3 receptor tyrosine kinase. FLT3LG is essential for B cell and dendritic cell (DC) development in both mice and humans 1. Notably, human FLT3LG is required for monocyte development, unlike its murine counterpart, and is only moderately required for NK cell development in humans, though it plays a more critical role in mouse NK cells 1. Mechanistically, FLT3LG functions through receptor binding on target immune cells. In tumor microenvironments, NK cells produce FLT3LG, which promotes DC differentiation and abundance; IL2 and IL15 signaling specifically induces NK cell FLT3LG production, enhancing anti-PD-1 immunotherapy efficacy 23. High FLT3LG expression correlates with increased infiltration of T cells and NK cells in lung adenocarcinoma 4. Clinically, FLT3LG variants are associated with Immunodeficiency 125, presenting with recurrent infections and hypoplastic bone marrow in homozygous loss-of-function carriers 1. In cancer contexts, elevated FLT3LG predicts favorable responses to immunotherapy and improved survival in colorectal cancer and lung adenocarcinoma patients 54. Additionally, FLT3LG exhibits strong colocalization evidence in autoimmune disease pathways 6.