Splenocytes collected from the immunized animal are cultured and fused by somatic hybridization (in the presence of polyethylene glycol or by electroporation) with murine immortal myeloma cells. The selection of fused cells (hybridomas) is performed by growing them on a selective medium containing HAT. On the one hand, this medium hinders the survival of immortal myeloma parents cells because these lack the HGPRT enzyme. On the other hand, non-fused B cells are unable to divide and eliminate themselves (even if they possess the HGPRT enzyme). Thus, only myeloma cells that have fused with the splenocytes can survive.