A monoclonal antibody is produced from a single clone of identical cells. To produce one, an antigen is injected into a mouse to stimulate antibody-producing lymphocytes. The lymphocytes are removed and fused with tumour cells to form hybridoma cells. The hybridomas are screened to identify the one producing the required antibody. This selected hybridoma is cloned to produce many genetically identical cells, which make the same antibody.