Hybridoma technology is used to mass-produce monoclonal antibodies that can only bind to a single epitope — part of an antigen that elicits an immune response. The process starts with injecting a mouse with a target antigen and then harvesting its spleen after a few weeks to isolate antibody-producing B cells. These cells have a limited life span and cannot be cultured in a lab. Next, the B cells are fused with myeloma cells — immortal and cancerous white blood cells, using electric pulses or polyethylene glycol. Thus, forming hybrid immortal, antibody-producing cells called hybridomas. Hybridomas are further selected on hypoxanthine -aminopterin -thymidine, or HAT medium based on the presence of a functional hypoxanthine-guanine phosphoribosyltransferase or HGPRT enzyme. In this medium, both the HGPRT negative myeloma cells and the HGPRT positive but short-lived B cells cannot survive for long. However, the immortal hybridoma cells with the HGPRT enzyme can proliferate. After selection, hybridoma cells that produce monoclonal antibodies with a strong affinity for the target epitope are identified and used as starter cells for establishing antibody-producing cell lines.