Mouse CD40 Ligand (CD40L), also known as CD154 and encoded by the Cd40lg gene, is a type II transmembrane protein belonging to the tumor necrosis factor (TNF) superfamily, which includes related ligands such as TNF-α, CD27 ligand (CD70), OX40 ligand (CD252), and 4-1BB ligand (CD137L) that regulate immune activation and cell–cell communication. In mice (Mus musculus), CD40L is primarily expressed on activated CD4⁺ T lymphocytes, and can also be detected on platelets and other activated immune cells following antigen stimulation. CD40L binds to its receptor CD40, which is expressed on B cells, dendritic cells, macrophages, and other antigen-presenting cells, initiating intracellular signaling pathways such as NF-κB and MAPK that promote B-cell activation, proliferation, immunoglobulin class switching, germinal center formation, and the development of long-lived memory B cells. Through these mechanisms, mouse CD40L plays a critical role in T cell–dependent antibody responses and regulation of adaptive immunity. Because mice are the most widely used genetic model organism in immunology, CD40L has been extensively studied in knockout and transgenic mouse models to investigate T cell costimulation, humoral immunity, autoimmune disease, and host responses to infection, providing key insights into CD40–CD40L signaling and its role in immune regulation and immunotherapy development.