The RM4-5 monoclonal antibody specifically reacts with mouse CD4, also known as L3T4, a 55 kDa differentiation antigen expressed by the majority of thymocytes, subpopulations of mature T cells (like major histocompatibility complex class II-restricted T lymphocytes), a subset of natural killer T cells, and on pluripotent hematopoietic stem cells. CD4 binds to the major histocompatibility complex class II (MHC class II) and enhances T lymphocyte development and mature T cells functions. In T lymphocytes, CD4 binds to the cytoplasmic tail of enzyme tyrosine kinase (p56lck).

Binding of RM4-5 is blocked by the anti-mouse CD4 clone GK1.5.