Synagis (palivizumab) is a humanized monoclonal antibody (IgG1?) produced by recombinant DNA technology, directed to an epitope in the A antigenic site of the F protein of respiratory syncytial virus (RSV). Synagis (palivizumab) is a composite of human (95%) and murine (5%) antibody sequences. The human heavy chain sequence was derived from the constant domains of human IgG1 and the variable framework regions of the VHgenes Cor (1) and Cess (2). The human light chain sequence was derived from the constant domain of C?and the variable framework regions of the VLgene K104 with J?-4 (3). The murine sequences were derived from a murine monoclonal antibody, Mab 1129 (4), in a process that involved the grafting of the murine complementarity determining regions into the human antibody frameworks. Synagis (palivizumab) is composed of two heavy chains and two light chains and has a molecular weight of approximately 148,000 Daltons.